White Paper: Using VPLEX Metro with VMware High Availability and Fault Tolerance for Ultimate Availability

White Paper

USING VPLEX™ METRO WITH VMWARE
HIGH AVAILABILITY AND FAULT
TOLERANCE FOR ULTIMATE AVAILABILITY

Abstract
This white paper discusses using best of breed technologies from
VMware® and EMC® to create federated continuous availability
solutions. The following topics are reviewed

 Choosing between federated Fault Tolerance or
federated High Availability
 Design considerations and constraints
 Operational Best Practice

September 2012

Copyright © 2012 EMC Corporation. All Rights Reserved.

EMC believes the information in this publication is
accurate as of its publication date. The information is
subject to change without notice.

The information in this publication is provided “as is.”
EMC Corporation makes no representations or
warranties of any kind with respect to the information in
this publication, and specifically disclaims implied
warranties of merchantability or fitness for a particular
purpose.

Use, copying, and distribution of any EMC software
described in this publication requires an applicable
software license.

For the most up-to-date listing of EMC product names,
see EMC Corporation Trademarks on EMC.com.

USING VMWARE FAULT TOLERANCE AND HIGH AVAILABILITY WITH 2
VPLEX™ METRO HA FOR ULTIMATE AVAILABILITY

Table of Contents
Executive summary ............................................................................................. 5
Audience ......................................................................................................................... 6
Document scope and limitations................................................................................. 6
Introduction .......................................................................................................... 8
EMC VPLEX technology ..................................................................................... 10
VPLEX terms and Glossary ........................................................................................... 11
EMC VPLEX architecture.............................................................................................. 13
EMC VPLEX Metro overview ........................................................................................ 14
Understanding VPLEX Metro active/active distributed volumes ........................... 15
VPLEX Witness – An introduction................................................................................. 18
Protecting VPLEX Witness using VMware FT .............................................................. 22
VPLEX Metro HA ............................................................................................................ 24
VPLEX Metro cross cluster connect ............................................................................ 24
Unique VPLEX benefits for availability and I/O response time ...................... 26
Uniform and non-uniform I/O access ........................................................................ 26
Uniform access (non-VPLEX) ....................................................................................... 26
Non-Uniform Access (VPLEX IO access pattern)...................................................... 31
VPLEX with cross-connect and non-uniform mode ................................................. 35
VPLEX with cross-connect and forced uniform mode ............................................ 36
Combining VPLEX HA with VMware HA and/or FT .......................................... 39
vSphere HA and VPLEX Metro HA (federated HA) .................................................. 39
Use Cases for federated HA ....................................................................................... 40
Datacenter pooling using DRS with federated HA.................................................. 40
Avoiding downtime and disasters using federated HA and vMotion .................. 41
Failure scenarios and recovery using federated HA ............................................... 42
vSphere FT and VPLEX Metro (federated FT) ............................................................ 45
Use cases for a federated FT solution ........................................................................ 45
Failure scenarios and recovery using federated FT ................................................. 46
Choosing between federated availability or disaster recovery (or both) ........... 49
Augmenting DR with federated HA and/or FT ......................................................... 51
Environments where federated HA and/or FT should not replace DR ................. 52
Best Practices and considerations when combining VPLEX HA with VMware
HA and/or FT....................................................................................................... 54
VMware HA and FT best practice requirements ...................................................... 55
Networking principles and pre-requisites .................................................................. 55
vCenter placement options ....................................................................................... 56


Path loss handling semantics (PDL and APD)........................................................... 57
Cross-connect Topologies and Failure Scenarios. ................................................... 58
Cross-connect and multipathing ............................................................................... 60
VPLEX site preference rules ......................................................................................... 60
DRS and site affinity rules ............................................................................................. 61
Additional best practices and considerations for VMware FT ............................... 61
Secondary VM placement considerations............................................................... 62
DRS affinity and cluster node count. ......................................................................... 63
VPLEX preference rule considerations for FT............................................................. 64
Other generic recommendations for FT .................................................................... 64
Conclusion ......................................................................................................... 66
References ......................................................................................................... 67
Appendix A - vMotioning over longer distances (10ms) .............................. 69


Executive summary
The EMC® VPLEX™ family removes physical barriers within, across, and
between datacenters. VPLEX Local provides simplified management and
non-disruptive data mobility for heterogeneous arrays. VPLEX Metro and
Geo provide data access and mobility between two VPLEX clusters within
synchronous and asynchronous distances respectively. With a unique
scale-out architecture, VPLEX’s advanced data caching and distributed
cache coherency provide workload resiliency, automatic sharing,
balancing and failover of storage domains, and enable both local and
remote data access with predictable service levels.
VMware vSphere makes it simpler and less expensive to provide higher
levels of availability for important applications. With vSphere, organizations
can easily increase the baseline level of availability provided for all
applications, as well as provide higher levels of availability more easily and
cost-effectively. vSphere makes it possible to reduce both planned and
unplanned downtime. The revolutionary VMware vMotion™ (vMotion)
capabilities in vSphere make it possible to perform planned maintenance
with zero application downtime.
VMware High Availability (HA), a feature of vSphere, reduces unplanned
downtime by leveraging multiple VMware ESX® and VMware ESXi™ hosts
configured as a cluster, to provide automatic recovery from outages as
well as cost-effective high availability for applications running in virtual
machines.
VMware Fault Tolerance (FT) leverages the well-known encapsulation
properties of virtualization by building fault tolerance directly into the ESXi
hypervisor in order to deliver hardware style fault tolerance to virtual
machines. Guest operating systems and applications do not require
modifications or reconfiguration. In fact, they remain unaware of the
protection transparently delivered by ESXi and the underlying architecture.
By leveraging distance, VPLEX Metro builds on the strengths of VMware FT
and HA to provide solutions that go beyond traditional “Disaster
Recovery”. These solutions provide a new type of deployment which
achieves the absolute highest levels of continuous availability over
distance for today’s enterprise storage and cloud environments. When
using such technologies, it is now possible to provide a solution that has
both zero Recovery Point Objective (RPO) with zero "storage" Recovery
Time Objective (RTO) (and zero "application" RTO when using VMware FT).
This white paper is designed to give technology decision-makers a deeper
understanding of VPLEX Metro in conjunction with VMware Fault Tolerance


and/or High Availability discussing design, features, functionality and
benefits. This paper also highlights the key technical considerations for
implementing VMware Fault Tolerance and/or High Availability with VPLEX
Metro technology to achieve "Federated Availability" over distance.

Audience
This white paper is intended for technology architects, storage
administrators and EMC professional services partners who are responsible
for architecting, creating, managing and using IT environments that utilize
EMC VPLEX and VMware Fault Tolerance and/or High Availability
technologies (FT and HA respectively). The white paper assumes that the
reader is familiar with EMC VPLEX and VMware technologies and
concepts.

Document scope and limitations
This document applies to EMC VPLEX Metro configured with VPLEX Witness.
The details provided in this white paper are based on the following
configurations:

• VPLEX Geosynchrony 5.1 (patch 2) or higher
• VPLEX Metro HA only (Local and Geo are not supported with FT or
HA in a stretched configuration)
• VPLEX Clusters are within 5 milliseconds (ms) of each other for
VMware HA
• Cross-connected configurations can be optionally deployed for
VMware HA solutions (not mandatory).
• For VMware FT configurations VPLEX cross cluster connect is in place
(mandatory requirement).
• VPLEX Clusters are within 5 millisecond (ms) round trip time (RTT) of
each other for VMware HA
• VPLEX Clusters are within 1 millisecond (ms) round trip time (RTT) of
each other for VMware FT
• VPLEX Witness is deployed to a third failure domain (Mandatory). The
Witness functionality is required for “VPLEX Metro” to become a true
active/active continuously available storage cluster.
• ESXi and vSphere 5.0 Update 1 or later are used
• Any qualified pair of arrays (both EMC and non-EMC) listed on the
EMC Simple Support Matrix (ESSM) found here:
https://elabnavigator.emc.com/vault/pdf/EMC_VPLEX.pdf


• The configuration is in full compliance with VPLEX best practice
found here:
http://powerlink.emc.com/km/live1/en_US/Offering_Technical/Tech
nical_Documentation/h7139-implementation-planning-vplex-tn.pdf

Please consult with your local EMC Support representative if you are
uncertain as to the applicability of these requirements.

Note: While out of scope for this document, it should be noted that in
addition to all best practices within this paper, that all federated FT and HA
solutions will carry the same best practices and limitations imposed by the
VMware HA and FT technologies too. For instance at the time of writing
VMware FT technology is only capable of supporting a single vCPU per VM
(VMware HA does not carry the same vCPU limitation) and this limitation
will prevail when federating a VMware FT cluster. Please ensure to review
the VMware best practice documentation as well as the limitations and
considerations documentation (please see the References section) for
further information.


Introduction
Increasingly, more and more customers wish to protect their business
services from any event imaginable that would lead to downtime.
Previously (i.e. prior to VPLEX) solutions to prevent downtime fell into two
camps:
1. Highly available and fault tolerant systems within a datacenter
2. Disaster recovery solutions outside of a datacenter.
The benefit of FT and HA solutions are that they provide automatic
recovery in the event of a failure. However, the geographical protection
range is limited to a single datacenter therefore not protecting business
services from a datacenter failure.
On the other hand, disaster recovery solutions typically protect business
services using geographic dispersion so that if a datacenter fails, recovery
would be achieved using another datacenter in a separate fault domain
from the primary. Some of the drawbacks with a disaster recovery
solutions, however, are that they are human decision based (i.e. not
automatic) and typically require a 2nd disruptive failback once the primary
site is repaired. In other words, should a primary datacenter fail the
business would need to make a non-trivial decision to invoke disaster
recovery.
Since disaster recovery is decision-based (i.e. manually invoked), it can
lead to extended outages since the very decision itself takes time, and this
is generally made at the business level involving key stakeholders. As most
site outages are caused by recoverable events (e.g. an elongated power
outage), faced with the “Invoke DR” decision some businesses choose not
to invoke DR and to ride through the outage instead. This means that
critical business IT services remain offline for the duration of the event.
These types of scenarios are not uncommon in these "disaster" situations
and non-invocation can be for various reasons. The two biggest ones are:
1. The primary site that failed can be recovered within 24-48 hours
therefore not warranting the complexity and risk of invoking DR.
2. Invoking DR will require a “failback” at some point in the future
which in turn will bring more disruption.
Other potential concerns to invoking disaster recovery include complexity,
lack of testing, lack of resources, lack of skill sets and lengthy recovery
time.
To avoid such pitfalls, VPLEX and VMware offer a more comprehensive
answer to safeguarding your environments. By combining the benefits of
HA and FT, a new category of availability is created. This new type of


category provides the automatic (non-decision based) benefits of FT and
HA, but allows them to be leveraged over distance by using VPLEX Metro.
This brings the geographical distance benefits normally associated with
disaster recovery to the table enhancing the HA and FT propositions
significantly.
The new category is known as “Federated Availability” and enables bullet
proof availability which in turn significantly lessens the chance of downtime
for both planned and unplanned events.


EMC VPLEX technology

VPLEX encapsulates traditional physical storage array devices and applies
three layers of logical abstraction to them. The logical relationships of each
layer are shown in Figure 1.
Extents are the mechanism VPLEX uses to divide storage volumes. Extents
may be all or part of the underlying storage volume. EMC VPLEX
aggregates extents and applies RAID protection in the device layer.
Devices are constructed using one or more extents and can be combined
into more complex RAID schemes and device structures as desired. At the
top layer of the VPLEX storage structures are virtual volumes. Virtual
volumes are created from devices and inherit the size of the underlying
device. Virtual volumes are the elements VPLEX exposes to hosts using its
Front End (FE) ports. Access to virtual volumes is controlled using storage
views. Storage views are comparable to Auto-provisioning Groups on EMC
Symmetrix® or to storage groups on EMC VNX®. They act as logical
containers determining host initiator access to VPLEX FE ports and virtual
volumes.

Figure 1 EMC VPLEX Logical Storage Structures


VPLEX terms and Glossary

Term Definition

VPLEX Virtual Unit of storage presented by the
Volume VPLEX front-end ports to hosts

VPLEX Distributed A single unit of storage presented by
Volume the VPLEX front-end ports of both
VPLEX clusters in a VPLEX Metro
configuration separated by
distance

VPLEX Director The central processing and
intelligence of the VPLEX solution.
There are redundant (A and B)
directors in each VPLEX Engine

VPLEX Engine Consists of two directors and is the
unit of scale for the VPLEX solution

VPLEX cluster A collection of VPLEX engines in one
rack.

VPLEX Metro The cooperation of two VPLEX
clusters, each serving their own
storage domain over synchronous
distance forming active/active
distributed volume(s)

VPLEX Metro HA As per VPLEX Metro, but configured
with VPLEX Witness to provide fully
automatic recovery from the loss of
any failure domain. This can also be
thought of as an active/active
continuously available storage
cluster over distance.

Access Anywhere The term used to describe a
distributed volume using VPLEX
Metro which has active/active
characteristics

Federation The cooperation of storage


elements at a peer level over
distance enabling mobility,
availability and collaboration

Automatic No human intervention whatsoever
(e.g. HA and FT)

Automated No human intervention required
once a decision has been made
(e.g. disaster recovery with
VMware's SRM technology)


EMC VPLEX architecture
EMC VPLEX represents the next-generation architecture for data mobility
and information access. The new architecture is based on EMC’s more
than 20 years of expertise in designing, implementing, and perfecting
enterprise-class intelligent cache and distributed data protection solutions.
As shown in Figure 2, VPLEX is a solution for vitalizing and federating both
EMC and non-EMC storage systems together. VPLEX resides between
servers and heterogeneous storage assets (abstracting the storage
subsystem from the host) and introduces a new architecture with these
unique characteristics:
• Scale-out clustering hardware, which lets customers start small and
grow big with predictable service levels
• Advanced data caching, which utilizes large-scale SDRAM cache to
improve performance and reduce I/O latency and array contention
• Distributed cache coherence for automatic sharing, balancing, and
failover of I/O across the cluster
• A consistent view of one or more LUNs across VPLEX clusters
separated either by a few feet within a datacenter or across
synchronous distances, enabling new models of high availability and
workload relocation

Physical Host Layer
A
A A A
A A

Virtual Storage Layer (VPLEX)
A

A

Physical Storage Layer

Figure 2 Capability of an EMC VPLEX local system to abstract
Heterogeneous Storage


EMC VPLEX Metro overview
VPLEX Metro brings mobility and access across two locations separated by
an inter-site round trip time of up to 5 milliseconds (host application
permitting). VPLEX Metro uses two VPLEX clusters (one at each location)
and includes the unique capability to support synchronous distributed
volumes that mirror data between the two clusters using write-through
caching.
Since a VPLEX Metro Distributed volume is under the control of the VPLEX
Metro advanced cache coherency algorithms, active data I/O access to
the distributed volume is possible at either VPLEX cluster. VPLEX Metro
therefore is a truly active/active solution which goes far beyond traditional
active/passive legacy replication solutions.
VPLEX Metro distributes the same block volume to more than one location
and ensures standard HA cluster environments (e.g. VMware HA and FT)
can simply leverage this capability and therefore can be easily and
transparently deployed and over distance too.
The key to this is to make the host cluster believe there is no distance
between the nodes so they behave identically as they would in a single
data center. This is known as “dissolving distance” and is a key deliverable
of VPLEX Metro.
The other piece to delivering truly active/active FT or HA environments is an
active/active network topology whereby the Layer 2 of the same network
resides in each location giving truly seamless datacenter pooling. Whilst
layer 2 network stretching is a pre-requisite for any FT or HA solution based
on VPLEX Metro, it is outside of the scope of this document. Going forward
throughout this document it is assumed that there is a stretched layer 2
network between datacenters where a VPLEX Metro resides.

Note: Please see further information on Cisco Overlay Transport
Virtualization (OTV) found here
http://www.cisco.com/en/US/docs/solutions/Enterprise/Data_Center/DCI/
whitepaper/DCI_1.html and Brocade Virtual Private LAN Service(VPLS)
found here
http://www.brocade.com/downloads/documents/white_papers/Offering_
Scalable_Layer2_Services_with_VPLS_and_VLL.pdf technology for
stretching a layer 2 network over distance.


Understanding VPLEX Metro active/active distributed volumes
Unlike traditional legacy replication where access to a replicated volume
is either in one location or another (i.e. an active/passive only paradigm)
VPLEX distributes a virtual device over distance which ultimately means
host access is now possible in more than one location to the same
(distributed) volume.
In engineering terms the distributed volumes that is presented from VPLEX
Metro is said to have “single disk semantics” meaning that in every way
(including failure) the disk will behave as one object as any traditional
block device would. This therefore means that all the rules associated with
a single disk are fully applicable to a VPLEX Metro distributed volume.
For instance, the following figure shows a single host accessing a single
JBOD type volume:

Datacenter
Figure 3 Single host access to a single disk

Clearly the host in the diagram is the only host initiator accessing the single
volume.
The next figure shows a local two node cluster.
Cluster of hosts coordinate for access

Datacenter
Figure 4 Multiple host access to a single disk

As shown in the diagram there are now two hosts contending for the single
volume. The dashed orange rectangle shows that each of the nodes is


required to be in a cluster or utilize a cluster file system so they can
effectively coordinate locking to ensure the volume remains consistent.
The next figure shows the same two node cluster but now connected to a
VPLEX distributed volume using VPLEX cache coherency technology.

Cluster of hosts coordinate
for access

VPLEX AccessAnywhere™

Datacenter Datacenter
Figure 5 Multiple host access to a VPLEX distributed volume

In this example there is no difference to the fundamental dynamics of the
two node cluster access pattern to the single volume. Additionally as far as
the hosts are concerned they cannot see any different between this and
the previous example since VPLEX is distributing the device between
datacenters via AccessAnywhere™ (which is a type of federation).
This means that the hosts are still required to coordinate locking to ensure
the volume remains consistent.
For ESXi this mechanism is controlled by the cluster file system Virtual
Machine File System (VMFS) within each datastore. In this case each
distributed volume will be imported into VPLEX and formatted with the
VMFS file system.
The figure below shows a high-level physical topology of a VPLEX Metro
distributed device.

A
A A A
A A

SITE A SITE B
AccessAnywhere™
A

LINK A

Figure 6 Multiple host access to a VPLEX distributed volume

This figure is a physical representation of the logical configuration shown in
Figure 5. Effectively, with this topology deployed, the distributed volume


can be treated just like any other volume, the only difference being it is
now distributed and available in two locations at the same time.
Another benefit of this type of architecture is “extreme simplicity” since it is
no more difficult to configure a cluster across distance that it is in a single
data center.

Note: VPLEX Metro can use either 8GB FC or native 10GB Ethernet WAN
connectivity (Where the word link is written). When using FC connectivity
this can be configured with either a dedicated channel (i.e. separate non
merged fabrics) or ISL based (i.e. where fabrics have been merged across
sites). It is assumed that any WAN link will have a second physically
redundant circuit.

Note: It is vital that VPLEX Metro has enough bandwidth between clusters
to meet requirements. EMC can assist in the qualification of this through
the Business Continuity Solution Designer (BCSD) tool. Please engage your
EMC account team to perform a sizing exercise.

For further details on VPLEX Metro architecture, please see the VPLEX HA Techbook
found here: http://www.emc.com/collateral/hardware/technical-
documentation/h7113-vplex-architecture-deployment.pdf


VPLEX Witness – An introduction
As mentioned previously, VPLEX Metro goes beyond the realms of legacy
active/passive replication technologies since it can deliver true
active/active storage over distance as well as federated availability.
There are three main items that are required to deliver true "Federated
Availability".
1. True active/active fibre channel block storage over distance.
2. Synchronous mirroring to ensure both locations are in lock step with
each other from a data perspective.
3. External arbitration to ensure that under all failure conditions
automatic recovery is possible.
In the previous sections we have discussed 1 and 2, but now we will look at
external arbitration which is enabled by VPLEX Witness.
VPLEX Witness is delivered as a zero cost VMware Virtual Appliance (vApp)
which runs on a customer supplied ESXi server. The ESXi server resides in a
physically separate failure domain to either VPLEX cluster and uses
different storage to the VPLEX cluster.
Using VPLEX Witness ensures that true Federated Availability can be
delivered. This means that regardless of site or link/WAN failure a copy of
the data will automatically remain online in at least one of the locations.
When setting up a single or a group of distributed volumes the user will
choose a “preference rule” which is a special property that each
individual or group of distributed volumes has. It is the preference rule that
determines the outcome after failure conditions such as site failure or link
partition. The preference rule can either be set to cluster A preferred,
cluster B preferred or no automatic winner.
At a high level this has the following effect to a single or group of
distributed volumes under different failure conditions as listed below:


Preference VPLEX CLUSTER PARTITION SITE A FAILS SITE B FAILS
Rule /
scenario Site A Site B Site A Site B Site A Site B

Cluster A ONLINE SUSPENDED FAILED SUSPENDED ONLINE FAILED
Preferred GOOD BAD (by design) GOOD

Cluster B SUSPENDED ONLINE FAILED ONLINE SUSPENDED FAILED
preferred GOOD GOOD BAD (by design)
No
automatic SUSPENDED (by design) SUSPENDED (by design) SUSPENDED (by design)
winner

Table 1 Failure scenarios without VPLEX Witness

As we can see in Table 1(above) if we only used the preference rules
without VPLEX Witness then under some scenarios manual intervention
would be required to bring the volume online at a given VPLEX cluster(e.g.
if site A is the preferred site, and site A fails, site B would also suspend).
This is where VPLEX Witness assists since it can better diagnose failures due
to the network triangulation, and ensures that at any time at least one of
the VPLEX clusters has an active path to the data as shown in the table
below:
Preference VPLEX CLUSTER PARTITION SITE A FAILS SITE B FAILS
Rule Site A Site B Site A Site B Site A Site B

Cluster A ONLINE SUSPENDED FAILED ONLINE ONLINE FAILED
Preferred GOOD GOOD GOOD

Cluster B SUSPENDED ONLINE FAILED ONLINE ONLINE FAILED
preferred GOOD GOOD GOOD
No
automatic SUSPENDED (by design) SUSPENDED (by design) SUSPENDED (by design)
winner

Table 2 Failure scenarios with VPLEX Witness

As one can see from Table 2 VPLEX Witness converts a VPLEX Metro from an
active/active mobility and collaboration solution into an active/active continuously
available storage cluster. Furthermore once VPLEX Witness is deployed, failure
scenarios become self-managing (i.e. fully automatic) which makes it extremely
simple since there is nothing to do regardless of failure condition!


Figure 7 below shows the high level topology of VPLEX Witness

Figure 7 VPLEX configured for VPLEX Witness

As depicted in Figure 7 we can see that the Witness VM is deployed in a
separate fault domain (as defined by the customer) and connected into
both VPLEX management stations via an IP network.

Note: Fault domain is decided by the customer and can range from
different racks in the same datacenter all the way up to VPLEX clusters 5ms
of distance away from each other (5ms measured round trip time latency
or typical synchronous distance). The distance that VPLEX witness can be
placed from the two VPLEX clusters can be even further. The current
supported maximum round trip latency for this is 1 second.


Figure 8 below shows a more detailed connectivity diagram of VPLEX
Witness

IMPORTANT /
REQUIREMENT!

SEPARATE FAULT
DOMAIN!

Figure 8 Detailed VPLEX Witness network layout

The witness network is physically separate from the VPLEX inter-cluster
network and also uses storage that is physically separate from either VPLEX
cluster. As stated previously, it is critical to deploy VPLEX Witness into a third
failure domain. The definition of this domain changes depending on where
the VPLEX clusters are deployed. For instance if the VPLEX Metro clusters
are to be deployed into the same physical building but perhaps different
areas of the datacenter, then the failure domain here would be deemed
the VPLEX rack itself. Therefore VPLEX Witness could also be deployed into
the same physical building but in a separate rack.
If, however, each VPLEX cluster was deployed 50 miles apart in totally
different buildings then the failure domain here would be the physical
building and/or town. Therefore in this scenario it would makes sense to
deploy VPLEX Witness in another town altogether; and since the maximum
round trip latency can be as much as one second then you could
effectively pick any city in the world, especially given the bandwidth
requirement is as low as 3Kb/sec.


For more in depth VPLEX Witness architecture details please refer to the
VPLEX HA Techbook that can be found here:
http://www.emc.com/collateral/hardware/technical-
documentation/h7113-vplex-architecture-deployment.pdf

Note: Always deploy VPLEX Witness in a 3rd failure domain and ensure that
all distributed volumes reside in a consistency group with the witness
function enabled. Also ensure that EMC Secure Remote Support (ESRS)
Gateway is fully configured and the witness has the capability to alert if it
for whatever reason fails (no impact to I/O if witness fails).

Protecting VPLEX Witness using VMware FT
Under normal operational conditions VPLEX Witness is not a vital
component that is required to drive active/active I/O (i.e. if the Witness is
disconnected or lost, I/O still continues).It does however become a crucial
component to ensure availability in the event of site loss at either of the
locations where the VPLEX clusters reside.
If, for whatever reason, the VPLEX Witness was lost and soon after there
was a catastrophic site failure at a site containing a VPLEX cluster then the
hosts at the remaining site would also lose access to the remaining VPLEX
volumes since the remaining VPLEX would think it was isolated as the VPLEX
Witness is also unavailable.
To minimize this risk, it is considered best practice to disable the VPLEX
Witness function if it has been lost and will remain offline for a long time.
Another way to ensure availability is to minimize the risk of a VPLEX Witness
loss in the first place by increasing the availability of the VPLEX Witness VM
running in the third location.
A way to significantly boost availability for this individual VM is to use
VMware FT to protect VPLEX Witness at the third location. This ensures that
the VPLEX Witness remains unaffected at the third failure domain should a
hardware failure occur to the ESXi server in the third failure domain that is
supporting the VPLEX Witness VM.
To deploy this functionality, simply enable ESXi HA clustering for the VPLEX
Witness VM across two or more ESXi hosts (in the same location), and once
this has been configured right click the VPLEX Witness VM and enable fault
tolerance.


Note: At the time of writing, the FT configuration on VPLEX Witness is only
within one location and not a stretched / federated FT configuration. The
storage that the VPLEX Witness uses should be physically contained within
the boundaries of the third failure domain on local (i.e. not VPLEX Metro
distributed) volumes. Additionally it should be noted that currently HA
alone is not supported, only FT or unprotected.


VPLEX Metro HA
As discussed in the two previous sections, VPLEX Metro is able to provide
active/active distributed storage, however we have seen that in some
cases depending on failure, loss of access to the storage volume could
occur if the preferred site fails for some reason causing the non-preferred
site to suspend too. Using VPLEX Witness overcomes this scenario and
ensures that access to a VPLEX cluster is always maintained regardless of
which site fails.
VPLEX Metro HA describes a VPLEX Metro solution that has also been
deployed with VPLEX Witness. As the name suggests VPLEX Metro HA
effectively delivers truly available distributed Storage volumes over
distance and forms a solid foundation for additional layers of VMware
technology such as HA and FT.

Note: It is assumed that all topologies discussed within this white paper use
VPLEX Metro HA (i.e. use VPLEX Metro and VPLEX Witness). This is
mandatory to ensure fully automatic (i.e. decision less) recovery under all
the failure conditions outlined within this document.

VPLEX Metro cross cluster connect
Another important feature of VPLEX Metro that can be optionally
deployed within a campus topology (i.e. up to 1ms) is cross cluster
connect.

Note: At the time of writing cross-connect is a mandatory requirement for
VMware FT implementations.

This feature pushes VPLEX HA into an even greater level of availability than
before since now an entire VPLEX cluster failure at a single location would
not cause an interruption to host I/O at either location (using either
VMware FT or HA)
Figure 9 below shows the topology of a cross-connected configuration:


A
A A
OPTIONAL A
A A

X – CONNECT
SITE A SITE B
AccessAnywhere™
A

LINK A

VPLEX
WITNESS

IP IP

Figure 9 VPLEX Metro deployment with cross-connect

As we can see in the diagram the cross-connect offers an alternate path or paths
from each ESXi server to the remote VPLEX.
This ensures that if for any reason an entire VPLEX cluster were to fail (which
is unlikely since there is no single-point-of-failure) there would be no
interruption to I/O since the remaining VPLEX cluster will continue to service
I/O across the remote cross link (alternate path)
It is recommended when deploying cross-connect that rather than
merging fabrics and using an Inter Switch Link (ISL), additional host bus
adapters (HBAs) should be used to connect directly to the remote data
centers switch fabric. This ensures that fabrics do not merge and span
failure domains.
Another important note to remember for cross-connect is that it is only
supported for campus environments up to 1ms round trip time.

Note: When setting up cross-connect, each ESXi server will see double the
paths to the datastore (50% local and 50% remote). It is best practice to
ensure that the pathing policy is set to fixed and mark the remote paths
across to the other cluster as passive. This ensures that the workload
remains balanced and only committing to a single cluster at any one time.


Unique VPLEX benefits for availability and I/O response
time
VPLEX is built from the ground up to perform block storage distribution over
long distances at enterprise scale and performance. One of the unique
core principles of VPLEX that enables this, is its underlying and extremely
efficient cache coherency algorithms which enable an active/active
topology without compromise.
Since VPLEX is architecturally unique from other virtual storage products,
two simple categories are used to easily distinguish between the
architectures.

Uniform and non-uniform I/O access
Essentially these two categories are a way to describe the I/O access
pattern from the host to the storage system when using a stretched or
distributed cluster configuration. VPLEX Metro (under normal conditions)
follows what is known technically as a non-uniform access pattern,
whereas other products that function differently from VPLEX follow what is
known as a uniform I/O access pattern. On the surface, both types of
topology seem to deliver active/active storage over distance, however at
the simplest level it is only the non-uniform category that delivers true
active/active within the non-uniform category which carries some
significant benefits over uniform type solutions.
The terms are defined as follows:
1. Uniform access
All I/O is serviced by the same single storage controller therefore all
I/O is sent to or received from the same location, hence the term
"uniform". Typically this involves "stretching" dual controller
active/passive architectures.
2. Non Uniform access
I/O can be serviced by any available storage controller at any given
location; therefore I/O can be sent to or received from any storage
target location, hence the term "non-uniform". This is derived from
"distributing" multiple active controllers/directors in each location.
To understand this in greater detail and to quantify the benefits of non-uniform
access we must first understand uniform access.

Uniform access (non-VPLEX)
Uniform Access works in a very similar way to a dual controller array that
uses an active/passive storage controller. With such an array a host would


generally be connected to both directors in a HA configuration so if one
failed the other one would continue to process I/O. However since the
secondary storage controller is passive, no write or read I/O can be
propagated to it or from it under normal operations since it remains
passive. The other thing to understand here is that these types of
architectures typically use cache mirroring whereby any write I/O to the
primary controller/director is synchronously mirrored to the secondary
controller for redundancy.
Next imagine taking a dual controller active/passive array and physically
splitting the nodes/controllers apart therefore stretching it over distance so
that the active controller/node resides in site A and the secondary
controller/node resides in site B.
The first thing to note here is that we now only have a single controller at
either location so we have already compromised the local HA ability of
the solution since each location now has a single point of failure.
The next challenge here is to maintain host access to both controllers from
either location.
Let's suppose we have an ESXi server in site A and a second one in site B. If
the only active storage controller resides at A, then we need to ensure that
hosts in both site A and site B have access to the storage controller in site A
(uniform access). This is important since if we want to run a host workload
at site B we will need an active path to connect it back to the active
director in site A since the controller at site B is passive. This may be
handled by a standard FC ISL which stretches the fabric across sites.
Additionally we will also require a physical path from the ESXi hosts in site A
to the passive controller at site B. The reason for this is just in case there is a
controller failure at site A, the controller at site B should be able to service
I/O.
As discussed in the previous section this type of configuration is known as
"Uniform Access" since all I/O will be serviced uniformly by the exact same
controller for any given storage volume, passing all I/O to and from the
same location. The diagram in Figure 10 below shows a typical example of
a uniform architecture.


Fabric A – Stretched via ISL

Fabric B Stretched via ISL A

A

Front End A

SPLIT CONTROLLERS Front End

Single Controller

Single Controller
A A
A A

Communication

Communication
(Active) (Passive)
Proprietary
Cache Cache
SITE A (Mirrored)
or
Dedicated (Mirrored)
SITE B
A
Backend ISL Backend A

A
A

(Mirrored) (Passive) A

Figure 10 A typical non-uniform layout

As we can see in the above diagram, hosts at each site connect to both
controllers by way of the stretched fabric; however the active controller
(for any given LUN) is only at one of the sites (in this case site A).
While not as efficient (bandwidth and latency) as VPLEX, under normal
operating conditions (i.e. where the active host is at the same location as
the active controller) this type of configuration functions satisfactorily,
however this type of access pattern starts to become sub-optimal if the
active host is propagating I/O at the same location where the passive
controller resides.
Figure 11 shows the numbered sequence of I/O flow for a host connected
to a uniform configuration at the local (i.e. active) site.
5



1 Front End
A
A
Front End
Single Controller

Single Controller

A A
A A

SPLIT CONTROLLERS
Communication

Communication

(Active) (Passive)
Cache Cache
SITE A (Mirrored)
All cache mirrored synchronously
(Mirrored)
SITE B
A
Backend 2 Backend A

A
A


3

4 4

Figure 11 Uniform write I/O Flow example at local site


The steps below correspond to the numbers in the diagram.
1. I/O is generated by the host at site A and sent to the active controller in site
A.

2. The I/O is committed to local cache, and synchronously mirrored to remote
cache over the WAN.

3. The local/active controller’s backend now mirrors the I/O to the back end
disks. It does this by committing a copy to the local array as well as sending
another copy of the I/O across the WAN to the remote array.

4. The acknowledgment from back end disk returns to the owning storage
controller.

5. Acknowledgement is received by the host and the I/O is complete.

Now, let's look at a write I/O initiated from the ESXi host at location B where
the controller for the LUN receiving I/O resides at site A.
The concern here is that each write at the passive site B will have to
traverse the link and be acknowledged back to site A. Before the
acknowledgement can be given back to the host at site B from the
controller at site A, the storage system has to synchronously mirror the I/O
back to the controller in site B (both cache and disk), thereby incurring
more round trips of the WAN. This ultimately increases the response time
(i.e. negatively impacts performance) and bandwidth utilization.
The numbered sequence in Figure 12 shows a typical I/O flow of a host
connected to a uniform configuration at the remote (i.e. passive) site.


1 6


2 Front End
A
A
Front End

Single Controller

Single Controller
A A
A A

SPLIT CONTROLLERS

Communication

Communication
(Active) (Passive)
Cache Cache
SITE A (Mirrored)
All cache mirrored synchronously
(Mirrored)
SITE B
A
Backend 3 Backend A

A
A


4

5 5

Figure 12 Uniform write I/O flow example at remote site

The following steps correspond to the numbers in the diagram.
1. I/O is generated by the host at site B and sent across the ISL to the
active controller at site A.
2. The I/O is received at the controller at site A from the ISL
3. The I/O is committed to local cache, and mirrored to remote cache
over the WAN and acknowledged back to the active controller in
site A.
4. The active controllers’ back end now mirrors the I/O to the back end
disks at both locations. It does this by committing a copy to the local
array as well as sending another copy of the I/O across the WAN to
the remote array (this step may sometimes be asynchronous).
5. Both write acknowledgments are sent back to the active controller
(back across the ISL)
6. Acknowledgement back to the host and the I/O is complete.

Clearly if using a uniform access device from a VMware datastore
perspective with ESXi hosts at either location, I/O could be propagated to
both locations perhaps simultaneously (e.g. if a VM were to be vMotioned
to the remote location leaving at least one VM online at the previous
location in the same datastore). Therefore in a uniform deployment, I/O
response time at the passive location will always be worse (perhaps
significantly) than I/O response time at the active location. Additionally,
I/O at the passive site could use up to three times the bandwidth of an I/O


at the active controller site due to the need to mirror the disk and cache
as well as send the I/O in the first place across the ISL.

Non-Uniform Access (VPLEX IO access pattern)
While VPLEX can be configured to provide uniform access, the typical
VPLEX Metro deployment uses non-uniform access. VPLEX was built from
the ground up for extremely efficient non-uniform access. This means it
has a different hardware and cache architecture relative to uniform
access solutions and, contrary to what you might have already read about
non-uniform access clusters, provides significant advantages over uniform
access for several reasons:
1. All controllers in a VPLEX distributed cluster are fully active. Therefore
if an I/O is initiated at site A, the write will happen to the director in
site A directly and be mirrored to B before the acknowledgement is
given. This ensures minimal (up to 3x better compared to uniform
access) response time and bandwidth regardless of where the
workload is running.
2. A cross-connection where hosts at site A connect to the storage
controllers at site B is not a mandatory requirement (unless using
VMware FT). Additionally, with VPLEX if a cross-connect is deployed,
it is only used as a last resort in the unlikely event that a full VPLEX
cluster has been lost (this would be deemed a double failure since a
single VPLEX cluster has no SPOFs) or the WAN has failed/been
partitioned.
3. Non-uniform access uses less bandwidth and gives better response
times when compared to uniform access since under normal
conditions all I/O is handled by the local active controller (all
controllers are active) and sent across to the remote site only once.
It is important to note that read and write I/O is serviced locally
within VPLEX Metro.
4. Interestingly, due to the active/active nature of VPLEX, should a full
site outage occur VPLEX does not need to perform a failover since
the remaining copy of the data was already active. This is another
key difference when compared to uniform access since if the
primary active node is lost a failover to the passive node is required.
The diagram below shows a high-level architecture of VPLEX when
distributed over a Metro distance:


Front End Front End Front End Front End

VPLEX Cluster B
Communication
A
A A

VPLEX Cluster A
A
A A

Communication
(Active) (Active) (Active) (Active)
Cache Cache Cache Cache
I P or
(Distributed) (Distributed) (Distributed) (Distributed)
FC

Backend
A

Backend Backend A Backend
A
A

A

SITE A SITE B

Figure 13 VPLEX non-uniform access layout

As we can see in Figure 13, each host is only connected to the local VPLEX
cluster ensuring that I/O flow from whatever location is always serviced by
the local storage controllers. VPLEX can achieve this because all of the
controllers (at both sites) are in an active state and able to service I/O.
Some other key differences to observe from the diagram are:
1. Storage devices behind VPLEX are only connected to each
respective local VPLEX cluster and are not connected across the
WAN, dramatically simplifying fabric design.
2. VPLEX has dedicated redundant WAN ports that can be connected
natively to either 10GB Ethernet or 8GB FC.
3. VPLEX has multiple active controllers in each location ensuring there
are no local single points of failure. With up to eight controllers in
each location, VPLEX provides N+1 redundancy.
4. VPLEX uses and maintains single disk semantics across clusters at two
different locations.
I/O flow is also very different and more efficient when compared to
uniform access too as the diagram below highlights.


4

1

VPLEX Cluster B
Communication
A
A A

VPLEX Cluster A
A
A A

Communication
2 Cache Cache
Cache Cache
(Distributed) (Distributed) Inter Cluster Com (Distributed) (Distributed)

Backend
A

A
A

A

3 3
SITE A SITE B

Figure 14 High level VPLEX non-uniform write I/O flow

The steps below correspond to the numbers in the Figure 14:
1. Write I/O is generated by the host at either site and sent to one of
the local VPLEX controllers (depending on path policy).
2. The write I/O is duplicated and sent to the remote VPLEX cluster.
3. Each VPLEX cluster now has a copy of the write I/O which is written
through to the backend array at each location. Site A VPLEX does
this for the array in site A, while site B VPLEX does this for the array in
site B.
4. Once the remote VPLEX cluster has acknowledged back to the local
cluster the acknowledgement is sent to the host and the I/O is
complete.

Note: Under some conditions depending on the access pattern, VPLEX
may encounter what is known as a local write miss condition. This does not
necessarily cause another step as the remote cache page owner is
invalidated as part of the write through caching activity. In effect, VPLEX is
able to accomplish several distinct tasks through a single cache update
messaging step.

The table below shows a broad comparison of the expected increase in
response time (in milliseconds) for I/O flow for both uniform and non-
uniform layouts if using an FC link with a 3 ms response time (and without
any form of external WAN acceleration / fast write technology). These


numbers are additional overhead when compared to a local storage
system of the same hardware, since I/O now has to be sent across the link.

(Based on 3ms RTT and 2 round trips per IO) SITE A Site B
Additional RT overhead (ms) read write read write
Full Uniform (sync mirror) 0 12 6 18
Full Uniform (async Mirror) 0 6 6 12
Non-Uniform (owner hit) 0 6* 0 6*
* This is comparable to standard synchronous Active/Passive replication

Key
Optimal
Acceptable, but not efficient
Sub-optimal

Table 3 Uniform vs. non-uniform response time increase

Note: Table 3 Only shows the expected additional latency of the IO on the
WAN and does not include any other overheads such as data
propagation delay or additional machine time at either location for
remote copy processing. Your mileage will vary.

As we can see in Table 3, topologies that use a uniform access pattern
and a synchronous disk mirror can add significantly more time to each I/O,
increasing the response time by as much at 3x compared to non-uniform.

Note: VPLEX Metro environments can also be configured using native IP
connectivity between sites. Using this type of topology caries further
response time efficiencies since each and every IO across the WAN only
typically incurs a single round trip.

Another factor to consider when comparing the two topologies is also the
amount of WAN bandwidth used. The table below shows a comparison
between a full uniform topology and a VPLEX non-uniform topology for
bandwidth utilization. The IO size example is 128KB and the results are also
shown in KB.


SITE A Site B
WAN bandwidth used for a 128KB IO read write read write
Full Uniform (sync or async mirror) 0 256 128 384
Non-Uniform 0 128* 0 128*
* This is comparable to standard synchronous Active/Passive replication

Key
Optimal
Acceptable, but not efficient
Sub-optimal

Table 4 Uniform vs. non-uniform bandwidth usage

As one can see from Table 4, non-uniform always performs local reads and
also only has to send the data payload once across the WAN for a write
I/O regardless of where the data was written. This is in stark contrast to a
uniform topology, especially if the write occurs at the site with the passive
controller, since now the data has to be sent once to across the WAN (ISL)
to the controller where it will both mirror the cache page (synchronously
over the WAN again)as well as mirror the underlying storage again back
over the WAN giving an overall 3x increase in WAN traffic when compared
to non-uniform.

VPLEX with cross-connect and non-uniform mode
When using VPLEX Metro with a cross cluster connect configuration (up to
1ms round-trip time) is sometimes referred to as "VPLEX in uniform mode"
since each ESXi host is now connected to both the local and remote
VPLEX clusters.
While on the surface this does look similar to uniform mode it still typically
functions in a non-uniform mode. This is because under the covers all
VPLEX directors remain active and able to serve data locally, maintaining
the efficiencies of the VPLEX cache coherent architecture. Additionally
when using cross-connected clusters, it is recommended to configure the
ESXi servers so that the cross-connected paths are only standby paths.
Therefore even with a VPLEX cross-connected configuration, I/O flow is still
locally serviced from each local VPLEX cluster and does not traverse the
link.
The diagram below shows an example of this:


Paths in standby


VPLEX Cluster B
Communication
A
A A

VPLEX Cluster A
A
A A

Communication
I P or
FC

Backend
A

A
A

A

SITE A SITE B

Figure 15 High-level VPLEX cross-connect with non-uniform I/O access

In Figure 15, each ESXi host now has an alternate path to the remote VPLEX
cluster. Compared to the typical uniform diagram in the previous section,
however, we can still see that the underlying VPLEX architecture differs
significantly since it remains identical to the non-uniform layout, servicing
I/O locally at either location.

VPLEX with cross-connect and forced uniform mode

Although VPLEX functions primarily in a non-uniform model, there are
certain conditions where VPLEX can sustain a type of uniform access
mode. One such condition is if cross-connect is used and certain failures
occur causing the uniform mode to be forced.
One of the scenarios where this may occur is when VPLEX and the cross-
connect network are using physically separate channels and the VPLEX
clusters are partitioned while the cross-connect network remains in place.
The diagram below shows an example of this:



VPLEX Cluster B
Communication
A
A A

VPLEX Cluster A
A
A A

Partition
Communication
(Active) (Active) (Passive) (Passive)

Backend
A

A
A

A

SITE A SITE B

Figure 16 forced uniform mode due to WAN partition

As illustrated in Figure 16 , VPLEX will invoke the "site preference rule"
suspending access to a given distributed virtual volume at one of the
locations (in the case site B). This ultimately means that I/O at site B has to
traverse the link to site A since the VPLEX controller path in site B is now
suspended due to the preference rule.
Another scenario where this might occur is if one of the VPLEX clusters at
either location becomes isolated or destroyed. The diagram below shows
an example of a localized rack failure at site B which has taken the VPLEX
cluster offline at site B.

VPLEX Cluster B
Communication

A
A A
VPLEX Cluster A

A
A A
Communication

(Active) (Active) (offline) (offline)
Localized
(Distributed) (Distributed)
I P or
FC
rack failure
(Distributed)(Distributed)

Backend
A

A
A

A

SITE A SITE B

Figure 17 VPLEX forced uniform mode due to cluster failure

In this scenario the VPLEX cluster remains online at site A (through VPLEX
Witness) and any I/O at site B will automatically access the VPLEX cluster at


site A over the cross-connect, thereby turning the standby path into an
active path.
In summary, VPLEX can use ‘forced uniform’ mode as a failsafe to ensure
that the highest possible level of availability is maintained at all times.

Note: Cross-connected VPLEX clusters are only supported with distances
up to 1 ms round trip time.


Combining VPLEX HA with VMware HA and/or FT
Due to its core design, EMC VPLEX Metro provides the perfect foundation
for VMware Fault Tolerance and High Availability clustering over distance
ensuring simple and transparent deployment of stretched clusters without
any added complexity.

vSphere HA and VPLEX Metro HA (federated HA)
VPLEX Metro takes a single block storage device in one location and
“distributes” to provide single disk semantics across two locations. This
enables a “distributed” VMFS datastore to be created on that virtual
volume.
On top of this, if the layer 2 network has also been “stretched” then a
single instance vSphere (including a single logical datacenter) can now
also be “distributed” into more than one location and HA enabled for any
given vSphere cluster! This is possible since the storage federation layer of
the VPLEX is completely transparent to ESXi. It therefore enables the user to
add ESXi hosts at two different locations to the same HA cluster.
Stretching a HA failover cluster (such as VMware HA) with VPLEX creates a
“Federated HA” cluster over distance. This blurs the boundaries between
local HA and disaster recovery since the configuration has the automatic
restart capabilities of HA combined with the geographical distance
typically associated with synchronous DR.

ESX
Distributed ESX HA Cluster
A
A A A

ESX
A A

VPLEX WAN VPLEX
A

A
A

A

Heterogeneous IP IP Heterogeneous
Storage VPLEX Storage
WITNESS

SITE A SITE B

Figure 18 VPLEX Metro HA with vSphere HA

For detailed technical setup instruction please see the VPLEX Procedure
generator - Configuring a distributed volume as well as the " VMware
vSphere® Metro Storage Cluster Case Study " white paper found here:


http://www.vmware.com/files/pdf/techpaper/vSPHR-CS-MTRO-STOR-
CLSTR-USLET-102-HI-RES.pdf for additional information around:
• Setting up Persistent Device Loss (PDL) handling
• vCenter placement options and considerations
• DRS enablement and affinity rules
• Controlling restart priorities (High/Medium/Low)

Use Cases for federated HA
A federated HA solution is an ideal fit if a customer has two datacenters
that are no more than 5ms (round trip latency) apart and wants to enable
an active/active datacenter design whilst also significantly enhancing
availability.
Using this type of solution brings several key business continuity items into
the solution including downtime and disaster avoidance as well as fully-
automatic service restart in the event of a total site outage. This type of
configuration would need to also be deployed with a stretched layer 2
network to ensure seamless capability regardless of which location the VM
runs in.

Datacenter pooling using DRS with federated HA
A nice feature of the federated HA solution is the ability for VMware DRS
(Dynamic Resource Scheduler) to be enabled and function relatively
transparently within the stretched cluster.
Using DRS effectively means that the vCenter/ESXi server load can be
distributed over two separate locations driving up utilization and using all
available, formerly passive, assets. Effectively with DRS enabled, the
configuration can be considered as two physical datacenters acting as a
single logical datacenter. This has some significant benefits since it brings
the ability to utilize what were once passive assets at a remote location
into a fully-active state.
To enable this functionality DRS can simply be switched on within the
stretched cluster and configured by the user to the desired automation
level. Depending on the setting, VMs will then automatically start to
distribute between the datacenters (Please read
http://www.vmware.com/files/pdf/techpaper/vSPHR-CS-MTRO-STOR-
CLSTR-USLET-102-HI-RES.pdf for more details).


Note: A design consideration to take into account if DRS is desired within a
solution is to ensure that there are enough compute and network
resources at each location to take the full load of the business services
should either site fail.

Avoiding downtime and disasters using federated HA and vMotion
Another nice feature of a federated HA solution with vSphere is the ability
to avoid planned downtime as well as unplanned downtime. This is
achievable using the vMotion ability of vCenter to move a running VM (or
group of VMs) to any ESXi server in another (physical) datacenter. Since
the vMotion ability is now federated over distance, planned downtime
can be avoided for events that affect an entire datacenter location.
For instance, let's say that we needed to perform a power upgrade at
datacenter A which will result in the power being offline for 2 hours.
Downtime can be avoided since all running VMs at site A can be moved
to site B before the outage. Once the outage has ended, the VMs can be
moved back to site A using vMotion while keeping everything completely
online.
This use case can also be employed for anticipated, yet unplanned
events.
For instance, a hurricane may be in close proximity to your datacenter, this
solution brings the ability to move the VMs elsewhere avoiding any
potential disaster.

Note: During a planned event where power will be taken offline it is best to
engage EMC support to bring the VPLEX down gracefully. However, in the
event of a scenario where time does not permit (perhaps a hurricane) it
may not be possible to involve EMC support. In this case if site A was
destroyed there would still be no interruption assuming the VMs were
vMotioned ahead of time since VPLEX Witness would ensure that the site
that remains online keeps full access to the storage volume once site A has
been powered off. Please see the Failure scenarios and recovery using
federated HA below for more details.


Failure scenarios and recovery using federated HA
This section addresses all of the different types of failures and shows how in
each case VMware HA is able to continue or restart operations ensuring
maximum uptime.
The configuration below is a representation of a typical federated HA
solution:

STRETCHED VSPHERE CLUSTER (DRS + HA)

ESX ESX
optional cross
connect
A
A A A
A A

SITE A SITE B

VPLEX VPLEX
A

A

WAN

IP IP

VPLEX
WITNESS

Figure 19 Typical VPLEX federated HA layout (multi-node cluster)

The table below shows the different failure scenarios and the outcome:

Failure VMs at A VMs at B Notes
Storage failure at Remain online / Remain online / Cache read miss at
site A uninterrupted uninterrupted sire A now incurs
additional link
latency, cache
read hits remain the
same as do write
I/O response times
Storage failure at Remain online / Remain online / Cache read miss at
site B uninterrupted uninterrupted site B now incurs
additional link
latency, cache
read hits remain the
same as do write
I/O response times
VPLEX Witness failure Remain online / Remain online / Both VPLEX clusters
uninterrupted uninterrupted dial home
All ESXi hosts fail at A All VMs are restarted Remain online / Once the ESXi hosts
automatically on are recovered, DRS


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