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1. Course materials may not be reproduced in whole or in part without the prior written permission of IBM.
Spectrum Scale Replication & Stretch
Clusters
© Copyright IBM Corporation 2015

2. Unit objectives
After completing this unit, you should be able to:
• Describe Replication
• Describe the Pros and Cons of Replication
• Describe a Stretch Cluster
© Copyright IBM Corporation 2015

3. Synchronous data replication
• Allows you to synchronously replicate
– A file, set of files or the entire file system
• Gives you better replication granularity as opposed to mirroring an entire
volume, which also saves on space used
– Allows you to replicate Metadata and/or data
– Provides additional layer(s) of protection in addition to the RAID level
protection of volumes underneath
– Supports a maximum of 3 copies of the data
– Replication is Synchronous only
– Asynchronous Replication can be achieved using AFM feature
© Copyright IBM Corporation 2015

4. Synchronous data replication
• To replicate or not replicate?
– This is Spectrum Scale level of replication which is an availability level
"on top of" the already built-in data availability (RAID) characteristics of
the disk subsystem(s) being used
– Can be used cross site
– Some performance impact.
Writes are 50 – 67% slower with replication levels 1 & 2
Reads are the same speed
– Your storage effectively becomes more expensive since you are using
more of your usable space for duplicate copies of your data
© Copyright IBM Corporation 2015

5. Data Replication Warnings
• If you decide to use Replication
– Always replicate your Metadata at the minimum
– Never replicate your data and not your metadata
• If you were to do this, then in the event of a failure, you would not be able
to mount your file system to retrieve your replicated data
© Copyright IBM Corporation 2015

6. Replication relies on failure groups
• Failure group
– A group of disks in a storage pool that Spectrum Scale assumes are
separate from the disks other failure groups.
– Can be changed anytime (mmrestripefs to fix data)
• A file is replicated when a copy of the data blocks exist in two
failure groups
– Ensures that no two replicas of the same block will become
unavailable due to a single failure.
• Can be set either at NSD creation time using the mmcrnsd
command or later on using the mmchdisk command.
• Important to set failure groups correctly to have effective file
system replication.
• Replication is per storage pool.
© Copyright IBM Corporation 2015

7. The third failure group
File System Descriptor Quorum
• In addition to quorum nodes three
disks, by default (NSDs) are used
as file system descriptors disks.
• A majority of the replicas on the
subset of disks must remain
available to sustain file system
operations.
• Spectrum Scale can move them
from one disk to another in case of
failure.
• Use the mmlsdisk –L command
to see the location of the descriptor.
• Can add one by creating a
descOnly NSD.
© Copyright IBM Corporation 2008
Disk Descriptor
Quorum
Node 1
Node 2

8. Replication/Failure groups and storage pools
• Creation of NSD requires [ mmcrnsd ]
– O/S disk name
– NSD Server List
• Optional, but recommended
– NSD name
– Failure Group (related to Replication)
– Storage Pool (related to Policy / ILM )
• Disk Stanza
%nsd:
device=/dev/sdav2
nsd=nsd1
servers=k145n06,k145n05
usage=dataOnly
failureGroup=5
pool=poolA
© Copyright IBM Corporation 2015

9. Accessing replicated data
• Default operation
– Read: read from all copies
– Write: write both copies
• Control with readReplicaPolicy
– Local: Read from block device or NSD server on same subnet
– Used for read heavy workload replicated across distance
• Operation with unavailable disk
– Disk marked “down” in FS descriptor
– Read: read available copy
– Write:
• Log changes for fast recovery (possible performance impact)
• Set “missing update” flag in the inode
• Write available copy
• Recovery
– Replay only changes to restored storage
© Copyright IBM Corporation 2015

10. Replication examples
• Full Replication
– 2 Failure Groups
– Data and Metadata
– On failure file system all ok
• Metadata Replication
– Replicate only metadata
– On failure data missing file system
stays mounted
Failure Group 1
Failure Group 2
Failure Group 1
Failure Group 2
Failure Group 3
Failure Group 4
inode
inode
Missing Data
Metadata OK
Failure Group 3 (Desc Only)
© Copyright IBM Corporation 2015

11. Mixing replication with pools
• Replicate only metadata
• 3 Data pools for capacity and single namespace
© Copyright IBM Corporation 2015

12. • Multi-site quorum configuration
• Replicate across sites
• Bandwidth requirements based on
application
• Often called - Two sites and a laptop
• Distributed data
– data is distributed across 2 sites, 3rd site contains
quorum node for availability
• Sites A and B
– Contain the core Spectrum Scale nodes and storage
– Multiple quorum nodes in each site
• Site C
– Contains a single quorum node
– Serves as tie breaker if one of the other sites
becomes inaccessible
– File System Descriptor NSD
Reliability: Multiple site High availability
Single Spectrum Scale System
Site A
Site B
Site C
WAN
© Copyright IBM Corporation 2015

13. Recovering from a storage failure
• Fix replication using mmrestripefs
mmrestripefs -R
• Usage:
mmrestripefs Device {-m | -r | -b | -p | -R} [-P PoolName]
[-N {Node[,Node...] | NodeFile | NodeClass}]
© Copyright IBM Corporation 2015

14. Review
• Replication can be on a single file or a whole file system
• Replication is spread across failure groups
• Replication is even more important when you do not have any
RAID support underneath for your volumes
• Replication is always synchronous
• Asynchronous Replication is covered by another feature called
Active File Management (AFM)
© Copyright IBM Corporation 2015

15. Course materials may not be reproduced in whole or in part without the prior written permission of IBM.
Spectrum Scale Stretch Clusters
© Copyright IBM Corporation 2015

16. Spectrum Scale Stretch Clusters
• Stretch Clusters combine two or more clusters together to
make one giant cluster
• Stretch Clusters are intended for inter-site or close proximity
clusters, not over WAN unless the amount of data is small
• Replication is not required, but is usually the intention for a
stretch cluster
• If replication between clusters is not the goal, then you might
possibly prefer using a multi-cluster set up.
• If replication is the goal, but it’s between data centers using a
WAN, then AFM may be a better choice if synchronous
replication is NOT required.
© Copyright IBM Corporation 2015

17. © 2013 IBM Corporation
Spectrum Scale Architecture
(Basics)
SAN, Shared SAS,
Twin Tailed, etc.
LUN = Logical Unit Number / NSD = Network Shared Disk
1
SAN LUN
Spectrum
Scale NSD
„1:1“ Relation

18. © 2013 IBM Corporation
Spectrum Scale Architecture
(Basics)
SAN
LUN = Logical Unit Number / NSD = Network Shared Disk
1a
SAN (etc) LUN
Spectrum
Scale NSD
„1:1“ Relation
Twin-tailed
SAS

19. © 2013 IBM Corporation
Spectrum Scale Architecture
(Common)
SAN
LAN
LUN‘s
Spectrum Scale NSD Client
Spectrum Scale NSD Server
2

20. © 2013 IBM Corporation
Spectrum Scale Architecture (Typical)
SAN
LAN / WAN / Infiniband & any Mixture
3
+ Twin-Tailed Disks
+ Internal Disks
FPO FPO
(FPO = File Placement Optimizer)
Spectrum Scale
replication of data on
disk
One (or multiple) filesystems
Files placed on different devices under policy control
Spectrum Scale NSD Clients
Spectrum Scale NSD Server
LUNs

21. © 2013 IBM Corporation
LUN‘s
NSD Clients
NSD Server
(NSD = Network Shared Disk)
LAN
Infiniband
remote cluster
Remote Cluster Mount (synchronous)
local cluster
4

22. © 2013 IBM Corporation
LUN‘s
NSD Clients
NSD Server
Inter-site LAN
Local LAN
Stretch Cluster (synchronous)
4a
Quorum node at 3rd site
Local LAN
Spectrum Scale
replication of data
between sites
Filesystem active across both sites
Site 2Site 1

23. © 2013 IBM Corporation
LUN‘s
NSD Clients
NSD Server
(NSD = Network Shared Disk)
WAN
Infiniband
remote cluster
Spectrum Scale Advanced File
Management (async)local cluster
Caching (R/W)
5

24. Exercise 4
Replication
Exercise
© Copyright IBM Corporation 2015

25. Unit summary
Having completed this unit, you should be able to:
• Describe replication
• Describe a Stretch Cluster
© Copyright IBM Corporation 2015