Considerations for Space Management when using HA/DR/Backup functions
As mentioned in last month’s entry,
Migration to Thin Provisioned volumes via PPRC provides a straightforward path to start taking advantage of this powerful function.
Thin Provisioning with small extents allows clients to efficiently allocate space automatically as new datasets are added or existing datasets are expanded, without having to expand the volumes themselves or change the configuration in any way.
In order to maintain maximum capacity efficiency, it is important that extents that become unused, due to dataset deletion, etc. are freed to be used for other purposes. The DFSMSdss™ SPACEREL command uses information on currently allocated datasets to communicate to the DS8K which extents can be freed.
This raises the question as to when SPACEREL should be run and how often does space need to be released? Each client understands the characteristics of their environment and may choose the frequency and schedule of releasing space based on some of the following criteria:
• Are there certain times of the day, or certain applications in which datasets are being quickly deleted and allocated?
o During these times or with certain applications, it may be counter-productive to release free space, as that will needlessly increase the number of space release and space allocation operations
• Are there threshold alerts being generated to indicate that Extent Pools are close to space exhaustion?
• Are there reporting / auditing requirements that require regular accurate representations of capacity consumed?
Another important factor in the management of Thin Provisioned volumes is the Copy Relations in which they are involved. Thin Provisioning Space Release is supported with DS8k Metro Mirror. When a SPACEREL command is issued to the Metro Mirror primary (production volume), space will also be released on the Metro Mirror secondary, if it is also a Thin Provisioned volume. This is a fairly straightforward operation as the Metro Mirror secondary is kept completely in sync with the Metro Mirror primary.
DS8K also offers many other mirroring choices including asynchronous replication such as Global Mirror as well as 3 and 4 site replication using Metro/Global Mirror, Metro/z Global Mirror, and Multi-target mirroring. In addition, Point in Time copies can be taken with FlashCopy® on its own or in combination with the various mirroring functions.
Asynchronous Replication and Point-in-Time copies present some unique challenges for Space Release. Space released on the production copies may not necessarily be immediately released on the mirrored copies or on PIT copies, as these copies represent a different time version than the production copies. Therefore, the actual Space Release in these environments may occur some time after the Space Release is requested.
IBM DS8K will continue to provide improved functionality to reduce complexity and increase efficiency in Thin Provisioned configurations, especially in High Availability and Disaster Recovery environments.