Migrating VDisks between MDisk groups using the CLI

You can migrate virtual disks (VDisks) between managed disk (MDisk) groups using the command-line interface (CLI).

You can determine the usage of particular MDisks by gathering input/output (I/O) statistics about nodes, MDisks, and VDisks. After you have gathered this data, you can analyze it to determine which VDisks or MDisks are hot. You can then migrate VDisks from one MDisk group to another.

Perform the following steps to gather statistics about MDisks and VDisks:

Isolate any VDisks that are overused. You can determine this by requesting an I/O statistics dump and analyzing the output.
To start I/O statistics gathering, issue the following CLI command:
```
svctask startstats -interval 15
```
This command generates a new I/O statistics dump file approximately every 15 minutes.
Wait for at least 15 minutes after issuing the svctask startstats command and then issue the following command:
```
svcinfo lsiostatsdumps
```
This command lists the I/O statistics files are generated on a per-node basis. These are prefixed with Nm for MDisk statistics, Nv for VDisk statistics, and Nn for node statistics.
Use secure copy (scp command) to retrieve the dump files for analyzing. For example, issue the following:
```
scp clusterip:/dumps/iostats/v_*
```
This copies all the VDisk statistics files to the AIX® host in the current directory.
Analyze the dumps to determine which VDisks are hot. It might be helpful to also determine which MDisks are being used heavily as you can spread the data that they contain more evenly across all the MDisks in the group by migrating the extents.
Stop the statistics collection again. Issue the following command:
```
svctask stopstats
```

After you analyze the I/O statistics data, you can determine which VDisks are hot. You also need to determine the MDisk group that you want to move this VDisk to. Either create a new MDisk group or determine an existing group that is not yet overly used. To do this, check the I/O statistics files that you generated and then ensure that the MDisks or VDisks in the target MDisk group are used less than those in the source group.

You can use data migration or VDisk mirroring to migrate data between MDisk groups. Data migration uses the command svctask migratevdisk. VDisk mirroring uses the commands svctask addvdiskcopy and svctask rmvdiskcopy.

When you issue the svctask migratevdisk command, a check is made to ensure that the destination of the migration has enough free extents to satisfy the command. If it does, the command proceeds. The command takes some time to complete.

Notes:

You cannot use the SAN Volume Controller data migration function to move a VDisk between MDisk groups that have different extent sizes.
Migration commands fail if the target or source VDisk is offline, or if there is insufficient quorum disk space to store the metadata. Correct the offline or quorum disk condition and reissue the command.

When you use data migration, it is possible for the free destination extents to be consumed by another process; for example, if a new VDisk is created in the destination MDisk group or if more migration commands are started. In this scenario, after all the destination extents are allocated, the migration commands suspend and an error is logged (error ID 020005). To recover from this situation, use either of the following methods:

Add additional MDisks to the target MDisk group. This provides additional extents in the group and allows the migrations to be restarted. You must mark the error as fixed before you reattempt the migration.
Migrate one or more VDisks that are already created from the MDisk group to another group. This frees up extents in the group and allows the original migrations to be restarted.

Perform the following steps to use the svctask migratevdisk command to migrate VDisks between MDisk groups:

After you determine the VDisk that you want to migrate and the new MDisk group you want to migrate it to, issue the following CLI command:
```
svctask migratevdisk -vdisk vdiskname/ID -mdiskgrp
 newmdiskgrname/ID -threads 4
```
You can check the progress of the migration by issuing the following CLI command:
```
svcinfo lsmigrate
```

When you use data migration, the VDisk goes offline if either MDisk group fails. VDisk mirroring can be used to minimize the impact to the VDisk because the VDisk goes offline only if the source MDisk group fails.

Perform the following steps to use VDisk mirroring to migrate VDisks between MDisk groups:

After you determine the VDisk that you want to migrate and the new MDisk group that you want to migrate it to, issue the following command:
```
svctask addvdiskcopy -mdiskgrp newmdiskgrname/ID vdiskname/ID
```
The copy ID of the new copy is returned. The copies now synchronize such that the data is stored in both MDisk groups. You can check the progress of the synchronization by issuing the following command:
```
svcinfo lsvdisksyncprogress 
```
After the synchronization is complete, remove the copy from the original I/O group to free up extents and decrease the utilization of the MDisk group. To remove the original copy, issue the following command:
```
svctask rmvdiskcopy -copy original copy id vdiskname/ID
```