Notes on migrating volumes

The system administrator can analyze the usage rate of disks, processor, and access paths for migrating volumes. However, you should keep in mind that Volume Migration operations can improve performance in one area while decreasing performance in another. Consider the following scenario:

Parity Group A has an average usage value of 20%
Parity Group B has an average usage value of 90%

Through Volume Migration, it is estimated that if one volume from Parity Group B is migrated to Parity Group A, the usage values of both parity groups become 55%. In this scenario, the I/O response time for Parity Group B will most likely decrease, while the I/O response time for Parity Group A will most likely increase. Overall, the throughput may increase or decrease.

Volume Migration must only be performed when you can expect a large improvement in storage system performance. Volume Migration operations may not provide significant improvement for cases in which parity group or volume usage varies only slightly, or for cases in which the overall MP usage is relatively high.

Note:

A Dynamic Provisioning virtual volume can be migrated to the following volumes:

External volume
Dynamic Provisioning virtual volume

When you migrate a Dynamic Provisioning virtual volume to an external volume, you cannot estimate the usage rate of the volume after migration because you cannot measure the usage rate.

When an error condition exists in the storage system, resource usage can increase or become unbalanced. Do not use data collected during an error condition as the basis for planning Volume Migration operations.

During firmware replacement, do not use a function that will not be supported after firmware replacement.

If you run a command for migrating volumes or waiting for events after you perform delete operation of a Volume Migration pair, take time (10 seconds recommended) until deletion of the pair is complete. Otherwise, the command might terminate abnormally.

If you migrate a volume or cancel migration of a volume by using CCI, the command might be rejected with [EX_CMDRJE] displayed, depending on the conditions of the DKC.

If you perform Volume Migration operations when data on the source volume is frequently updated, make sure that the setting of the following system option modes is appropriate.

System option mode 899 (ON by default)

System opiton mode 900 (OFF by default)

Behavior

OFF

When data on the source volume is updated, an asynchronous copy to the target volume is performed.¹

OFF

ON

When data on the source volume is updated, an asynchronous copy is performed. However, if the number of retries of the asynchronous copy exceeds the half of the retry threshold³ for asynchronous copy, the copy mode is switched to synchronous copy.

ON

OFF

When data on the source volume is updated, an asynchroous copy is performed. However, if the number of retries of the asynchronous copy exceeds the retry threshold³ for asynchronous copy, the copy mode is switched to synchronous copy.

ON

When data on the source volume is updated, a synchronous copy to the target volume is performed.²

Notes:

In this case, the influence on the I/O performance can be suppressed, but volume migration takes time.
In this case, volume migration takes less time, but the influence on the I/O performence is larger.

The following table describes the retry threshold for asynchronous copy.

Number of slots* of the source volume	Retry threshold (times)
0 < Number of slots of the source volume ≤ 4,0960	20
40,960 < Number of slots of the source volume ≤ 409,600	50
409,600 < Number of slots of the source volume ≤ 4,194,304	40
4,194,304 < Number of slots of the source volume	50
* For details about the number of slots, see the VSP One Block Provisioning Guide.

When you migrate a volume, the MP unit assigned to the LDEV used as the target volume is reassigned to the MP unit assigned to the source volume.

When you start migrating a volume, the MP unit assigned to the target volume is changed. If the migration is complete or interrupted, the volume migration pair is deleted, and then the MP unit that was assigned to the target volume before migration is reassigned to the target volume.

However, in either of the following cases, the former MP unit is not reassigned to the target volume:

A user changes the assignment of the MP unit to the source or target volume.
When the volume migration is complete or interrupted and the Volume Migration pair is deleted, the cache write pending rate of the MP unit assigned to the target volume is 50% or more, or the cache write pending rate of the former MP unit is 50% or more.

Also, when the former MP unit is removed, another MP unit is assigned to the target volume.