Latency issues when compressed and uncompressed volumes are created in same storage pool on SVC/V7000

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Problem(Abstract) – Reference #S1004312

Creating compressed and uncompressed volumes in the same Storage Pool/mdisk group may result in latency issues.

Symptom

High response times on volumes, which affects host performance

Cause

It is recommended that compressed and uncompressed volumes be created in separate storage pools/mdisk groups as failure to separate them may result in increased response times or lower cache destage rates for the uncompressed volumes.

Mixing the volume types will cause the compressed and uncompressed volumes to share the same cache partition. When hosts are sending write IOs, if response time from backend increases above a certain level, the cache destaging to the whole storage pool is throttled down, and the cache partition will become full.

Since compression adds latency, lower compression volumes and/or heavy write scenarios may cause the cache partition to be put on hold more than desired. The symptom will be a full cache partition and higher latency for the volumes in the partition.

Resolving the problem

Creating compressed and uncompressed volumes in separate storage pools/mdisk groups will reduce the impact on uncompressed volumes.

An enhancement will be added in a future release of the SVC/V7000 to reduce the impact on uncompressed volumes.

Note: The first priority should be to validate that the compression ratio for volumes is 45% or higher. Compression adds latency and volumes with low compression ratio adds even more latency, which can lead to cache full conditions. Migrating volumes with compression ratio below 45% to uncompressed volumes may mitigate the need for separate storage pools in certain cases.

Also see:

http://www-01.ibm.com/support/docview.wss?uid=ssg1S1004309

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