Migrating VDisks

You can migrate a virtual disk (VDisk) from one managed disk (MDisk) group to another from the Migrating VDisks panel.

This task assumes that you have already launched the SAN Volume Controller Console.

The SAN Volume Controller provides various data migration features. You can use these features to move the placement of data both within MDisk groups and between MDisk groups. These features can be used concurrently with I/O operations. There are two ways that you can migrate data:

Migrate data (extents) from one MDisk to another MDisk within the same MDisk group. This can be used to remove active or overutilized MDisks. This can only be performed using the command-line interface (CLI).
Migrate VDisks from one MDisk group to another. This can be used to remove active MDisk groups; for example, you can reduce the utilization of a group of MDisks.

You can determine the usage of MDisks by gathering 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 active.

When a migration command is issued, a check ensures that the migration target has sufficient free extents available. If there are sufficient free extents, the command proceeds.

Notes:

You cannot use the SAN Volume Controller data migration function to move a VDisk between MDisk groups that have different extent sizes. However, you can start with a non-mirrored VDisk in one MDisk group and then add a mirrored copy to that VDisk in another MDisk group. You can also create a FlashCopy® mapping to create an instant copy of a VDisk that is in another MDisk group.
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 retry the command.

While the migration proceeds, it is possible for the free destination extents to be consumed by another process; for example, by creating a new VDisk in the destination MDisk group or by starting more migrate commands. In this situation, when all the destination extents have been allocated, the migration commands suspend and an error is logged (error ID 020005). There are two methods for recovering from this situation:

Add additional MDisks to the target MDisk group. This provides additional extents in the group and allows the migrations to be restarted (by marking the error as fixed).
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 migrate VDisks between MDisk groups:

Perform the following steps to determine if VDisks are overused:
1. Click Manage Cluster > Start statistics collection in the portfolio. The Starting the Collection of Statistics panel is displayed.
2. Enter 15 minutes for the interval and click OK. This generates a new I/O statistics dump file approximately every 15 minutes.
3. Wait at least 15 minutes before you proceed to the next step.
View the I/O statistics log.
1. Click Service and Maintenance > List dumps in the portfolio. The List Dumps panel is displayed.
2. Click I/O Statistics Logs. This lists the I/O statistics files that have been generated on a per-node basis. These are prefixed with Nm for MDisk statistics, Nv for VDisk statistics, and Nn for node statistics.
3. Click a filename to view the contents of the log.
4. Analyze the dumps to determine which VDisks are active. It might be helpful to also determine which MDisks are heavily utilized so you can spread the data that they contain more evenly across all the MDisks in the group. Either create a new MDisk group or determine an existing group that is not yet over used. You can do this by checking the I/O statistics files that were previously generated and ensuring that the MDisks or VDisks in the target MDisk group are less utilized than the source group.
Stop the statistics collection by clicking Manage Cluster > Stop statistics collection in the portfolio.
Migrate the VDisk.
1. Click Work with Virtual Disks > Virtual Disks in the portfolio. The Viewing Virtual Disks panel is displayed.
2. Select the VDisk to migrate and select Migrate a VDisk from the task list. Click Go. The Migrating Virtual Disks panel is displayed.
3. Select a target MDisk group from the Target MDisk Group list.
4. Click OK.