Exploring Stretched Clusters

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© Copyright 2012 EMC Corporation. All rights reserved.

EXPLORING STRETCHED
CLUSTERS
Examining the use of stretched clusters in a
VMware vSphere environment

Scott Lowe, VCDX 39
CTO, VMware Affinity Team
Author, Mastering VMware vSphere 5
Blogger, http://blog.scottlowe.org


Part 1: Stretched Cluster
Or SRM?


Part 1 Agenda

• Quick review of terminology
• Comparing SRM and vMSC requirements
• Comparing SRM and vMSC advantages
• Comparing SRM and vMSC disadvantages
• Mixing SRM and vMSC


New vMSC HCL category

• vMSC = vSphere Metro Stretched Cluster
• Introduces some new terms:
• Uniform access = "stretched SAN"
• Non-uniform access = "distributed virtual storage"
• Provides boundaries for supportability of
stretched cluster configurations


RPO versus RTO

• RPO = Recovery Point Objective
• RPO is a measure of how much data loss the
organization is willing to sustain
• RTO = Recovery Time Objective
• RTO is a measure of how long of a wait the
organization is willing to tolerate before
recovery is complete


DR versus DA

• DA = Disaster avoidance
• Seeks to protect applications and data before a
disaster occurs
• How often do you know before a disaster is going to
occur?
• DR = Disaster recovery
• Seeks to recover applications and data after a
disaster occurs
• Think of DA as vMotion and DR as vSphere HA


Requirements for SRM

• Some form of supported storage replication
(synchronous or asynchronous)
• Layer 3 connectivity
• No minimum inter-site bandwidth
requirements (driven by SLA/RPO/RTO)
• No maximum latency between sites (driven by
SLA/RPO/RTO)
• At least two vCenter Server instances


Requirements for vMSC

• Some form of supported synchronous active/
active storage architecture
• Stretched Layer 2 connectivity
• 622Mbps bandwidth (minimum) between sites
• Less than 5 ms latency between sites (10 ms
with vSphere 5 Enterprise Plus/Metro vMotion)
• A single vCenter Server instance


Advantages of SRM

• Defined startup orders (with prerequisites)
• No need for stretched Layer 2 connectivity
(but supported)
• The ability to simulate workload mobility
without affecting production
• Supports multiple vCenter Server instances
(including in Linked Mode)


Advantages of vMSC

• The possibility of non-disruptive workload
migration (disaster avoidance)
• No need to deal with issues changing IP
addresses
• Potential for running active/active data centers
and more easily balancing workloads between
them
• Typically a near-zero RPO with RTO of minutes
• Requires only a single vCenter Server instance


Disadvantages of SRM

• Typically higher RPO and RTO than stretched
clusters
• Workload mobility is always disruptive
• Requires at least two vCenter Server instances
• Operational overhead from managing
protection groups and protection plans


Disadvantages of vMSC

• Greater physical networking complexity due to
stretched Layer 2 connectivity requirement
• Greater cost resulting from higher-end
networking equipment, more bandwidth,
active/active storage solution
• No ability to test workload mobility
• Operational overhead from management of
DRS host affinity groups
• Supports only a single vCenter Server instance


What about a mixed
architecture?

• It can be done, but it has its own set of design
considerations
• For any given workload, it's an “either/or”
situation


Diagram of a mixed
architecture


Part 2: Building Stretched
Clusters


Part 2 Agenda

• vSphere recommendations
• Storage recommendations
• Networking recommendations
• Operational recommendations


vSphere recommendations

• Use vSphere 5
• Use DRS host affinity groups
• Run vSphere HA with N+2 capacity


Use vSphere 5

• vSphere 5 eliminates some vSphere HA
limitations
• vSphere 5 introduces the vMSC HCL category


Use vSphere DRS host affinity
groups

• Allows you to mimic site awareness
• Use PowerCLI to address manageability
concerns
• Use "should" rules instead of "must" rules


Using PowerCLI with host
affinity groups

• Use some sort of unique property to "group"
VMs
• Use this "grouping" to automate VM placement
into groups
• Run the PowerCLI script regularly to ensure
correct VM group assignment


Storage recommendations

• Use storage from vMSC category
• Be aware of storage performance
considerations
• Account for storage availability
• Plan Storage DRS carefully
• Use profile-driven storage


Account for storage
availability

• Consider cross-connect topology
• Ensure multiple storage controllers at each site
for availability
• Provide redundant and independent inter-site
storage connections
• With VPLEX, use the third-site cluster witness


Plan Storage DRS carefully

• Align datastore clusters to site/array
boundaries
• Don't combine stretched/non-stretched
datastores
• Understand the impact of SDRS on your
storage solution


Use profile-driven storage

• Use user-defined capabilities to model site
topology
• Create VM storage profiles to provide site
affinity
• Can help avoid operational concerns with VM
placement


Networking recommendations

• Plan for different traffic patterns
• Where possible, separate management traffic
onto a vSwitch
• Incorporate redundant and independent inter-
site network connections
• Minimize latency as much as possible


Operational recommendations

• Account for backup/restore in your design
• Handle inter-site vMotion carefully
• Don't split multi-tier apps across sites


Backup/restore for stretched
clusters

• Consider a solution with client-side
deduplication to reduce WAN traffic
• A mechanism to reduce restore traffic would
be nice to have as well
• Might be able to leverage storage solution
itself for restores
• Restore to local side
• Allow storage solution to replicate to remote side


Handling inter-site vMotion

• An inter-site vMotion will impact DRS host
affinity rules
• An inter-site vMotion could require storage
configuration updates
• Review inter-site vMotions to:
• Reconcile DRS host affinity rules and VM locations
• Reconcile storage availability rules and VM locations
• Impact on other operational areas


Questions & Answers


Exploring Stretched Clusters

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