SnapMirror Basics:
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1. Firstly, Snapshot technology is the foundation for SnapMirror
2. SnapMirror is a fast and highly flexible solution for replicating data over LAN, WAN and FC networks. It can also be used for data migration, data distribution for load balancing and remote access.
3. No interruption to data access while migrating data from one storage system to another.
4. There are three types of SM:
a. Asynchronous (Incremental updates based on schedules)
i. Volume SnapMirror (VSM)
ii. Qtree SnapMirror (QSM)
b. Synchronous (fully synchronous incremental updates are instantaneous)
i. Only applies to VSM
ii. Replicates data with little or no lag, depending on network latency we can have performance issues.
c. Semi-synchronous (almost instantaneous upon receipt of a write request on the source)
VSM key points:
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1. Block-for-block replication
2. Supports async, sync and semi-sync modes
3. No performance effect even with large or deep directories
4. Source volumes are online writable, destination volumes are online read-only
5. Quotas cannot be enabled on destination volumes
6. Transfers between traditional and flexible volumes are not supported
7. From ONTAP 8.1 7-mode, transfers between 32-bit and 64-bit aggregates are supported
VSM Mechanism:
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1. Initial (baseline) transfer to a "restricted" destination volume, during which the source storage system creates a snapshot copy of the volume. All data blocks that are referenced by the snapshot copy, including volume metadata such as language translation settings and all snapshot copies of the volume, are transferred and written to destination volume.
2. Scheduled updates, these are configured in the "snapmirror.conf" file. After initialization is completed, the source and destination file system share one snapshot copy, there after the updates occur as scheduled in the "snapmirror.conf" file.
QSM key points:
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1. QSM is a logical replication, all files and directories in the source file system are created in the destination qtree
2. Source and destination volumes can be traditional or flexible, volume size and disk geometry are irrelevant
3. Qtrees from multiple sources can be replicated to one destination
4. This is little bit tricky, with QSM the source and destination volume can be online and writable, but the destination qtree is not writable.
5. Supports only Async mode
6. Cannot initialize to tape, does not support SM cascades, deep directory structures and large number of files might affect performance
QSM Mechanism:
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1. Initial (baseline) transfer, creates the destination qtree automatically. QSM creates a snapshot copy of source volume including all its data and metadata.
2. Scheduled updates: after the baseline transfer is completed the source and destination filesystems share a snapshot copy and thereafter the updates occur as scheduled in the "snapmirror.conf" file.
3. QSM does not transfer the volume or the snapshot copy to the destination, rather it transfers only the qtree data that is new or has changed.
Now that we have gone over some key concepts, I will now go through the configuration process.
SnapMirror Configuration:
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Step:1 Add SnapMirror license
filerA > license add <license>
filerB > license add <license>
Step:2 Set SnapMirror access
filerA > options snapmirror.access host=filerB
filerB > options snapmirror.access host=filerA
Step:3 Set configuration file
Syntax: "Source-storage-system name:source-path" "destination-storage-system:destination-path" "arguments" "schedule"
Note: 1. Please don't put quotes, follow the example format shown below.
2. Arguments can be left default or can set some thing like speed kbs=2000
3. Schedule follow a format like this: 10 22 * 1,3,4
10 - Update 10 minutes past hour
22 - Update at 10 pm
* - Update on all applicable days of the month
1,3,4 - Update on Monday, Wednesday, Thursday
QSM
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filerB > wrfile /etc/snapmirror.conf
filerA:/vol/volA/q1 filerB:/vol/volA/q1 - 10 * * *
In the above example I meant the source filer (filerA), Qtree (q1) inside Volume (volA) to replicate to destination filer (filerB), Qtree (q1) inside Volume (volA) with no arguments specified are scheduled to run every day at every 10minutes past the hour.
VSM:
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filerB > wrfile /etc/snapmirror.conf
filerA:/vol/volA filerB:/vol/volA kbs=3000 15 7,19 * *
n the above example I meant the source filer (filerA),Volume (volA) be replicated to destination filer (filerB), Volume (volA) to use maximum of 3,000 kilobytes per second to transfer data and is scheduled to run every day at every 15minutes past 7 am and also at 15 minutes past 7 pm (meaning the update to run at 7:15 am and 7:15 pm every day of the month and every day if the week).
Step:4 Perform Baseline transfer
Note: The destination volume must be restricted
filerB > vol restrict volA
filerB > snapmirror initialize -S filerA:volA filerB:volA
Step:5 Monitor
filerB > snapmirror status <options>
options: -l ( displays the long format of the output)
-q ( displays the volumes or qtrees that are quiesced or quiescing)
Note: Never delete the snapshot copies that are created by snapmirror. If snapmirror cannot find the copy on the source, it cannot perform incremental changes to the destination. The affected relationship must be reinitialized.
Log files:
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Check to see if the snapmirror logging is enabled from "options.snapmirror.log". If enabled you should find logs at "/etc/logs/snapmirror"
Managing Transfers:
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One can specify the number of volume snapmirror transfers to reserve resources. This option means the resources reserved for VSM transfers are not available for other replication types like QSM or Snapvault transfers.
options replication.volume.reserved_transfers <n>
n - number of VSM transfers for which you want to reserve resources, default is 0
Stream-count setting can be modified to previous by using
options replication.volume.transer_limits [current|previous]
Network Throttling:
===============
1. One can use per-transfer throttling by specifying arguments in the "snapmirror.conf" file as shown above with "kbs"
2. Dynamic throttling enables to change throttle value while the transfer is active
snapmirror throttle <n> destination-hostname:destination-path
3. System-wide throttling limits the amount of bandwidth that can be used for transfers
options replication.throttle.enable on
options replication.throttle.incoming.max_kbs
options replication.throttle.outgoing.max_kbs
Default is unlimited, this can be changed between 1 to 125,000 kbs
Snapmirror Network Compression:
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1. Snapmirror has a built-in feature that enables data compression over the network for SM transfers. This is different from WAFL compression, which compresses data at rest.
2. Compressed data is sent to and received by the destination filer, the destination filer than decompressed and saves to appropriate volume.
Note: Supported only on asynchronous mode of VSM
Breaking SnapMirror relationship:
===========================
One can break a SnapMirror relationship to convert a replica to a writable file system, do it from destination filer
filerB > snapmirror break <volume name>
To resume the SM relationship you can use the following command from destination filer
filerB > snapmirror resync filerB:volume-name
Note: In case "real" disaster the process is very different, please follow the scenario in Example 2
To break a relationship permanently execute the following from source filer
filerA > snapmirror release source_vol filerB:dest_vol
Example:1
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Scenario: To create and schedule asynchronous VSM relationship between filerA (source), volA and filerB (destination), volA-mirror.
Step:1 Configure access on both systems
filerA > options snapmirror.access host=filerB
filerB > options snapmirror.access host=filerA
Step:2 Create 100-MB flex volume with name volA_mirror in aggr1
filerB > vol create volA_mirror aggr1 100m
Step:3 Set Security style
filerB > qtree security /vol/volA_mirror ntfs
Step:4 Create CIFS share
filerB > cifs shares -add volA_mirror /vol/volA_mirror
Step:5 Initiate baseline transfer (Remember to restrict the destination volume)
filerB > vol restrict volA_mirror
filerB > snapmirror initialize -S filerA:volA filerB:volA_mirror
Step:6 Monitor and verify if the baseline transfer is completed
filerB > snapmirror status
filerA > snap list volA
filerB > snap list volA_mirror
Step:7 If you want to update destination volume, you can always do
filerB > snapmirror update -S filerA:volA filerB:volA_mirror
Step:8 Schedule snapmirror by setting the "snapmirror.conf" file
filerB > wrfile /etc/snapmirror.conf
filerA:volA filerB:volA_mirror - 10 * * *
Example:2
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Scenario: To simulate a disaster recovery and failover to the mirror
Step:1 To simulate disaster, bring the source volume offline
filerA > vol offline volA
Step:2 Break relationship from destination
filerB > snapmirror quiesce volA_mirror
filerB > snapmirror break volA_mirror
Resynchronize the SM relationship
Because new data was written to the destination during the disaster, the destination storage system is now acts as source. Lets resynchronize new data back to original volume
Step:3 Bring the volA back online
filerA > vol online volA
Step:4 From filerA replicate the new data that was written to filerB:/volA_mirror during the disaster back to filerA:/volA
filerA > snapmirror resync -S filerB:volA_mirror filerA:volA
Step:5 After the resync is completed break the reverse relationship
filerA > snapmirror quiesce volA
filerA > snapmirror break volA
Reinstate the original filerA-to-filerB SM relationship
Step:6 Now resync from filerB to return to the original source-to-destination relationship
filerB > snapmirror resync filerB:volA_mirror
Step:7 Test the operation by creating a file of filerA:/volA and then using snapmirror update command from filerB
filerB > snapmirror update volA_mirror
