DS8000 Extent Space Efficient Volumes

Using Extent Space Efficient volumes with the DS8000 Storage System is a way to reduce storage costs and reduce the storage footprint.  It allows you to create and use volumes where the sum of all the virtual volume capacity is greater than the physical storage of the extent pool.  Physical storage is allocated only as needed.  When datasets are moved or deleted, some of the physically provisioned storage may no longer be associated with a current dataset and becomes logically free.

The DFSMSdss SPACEREL command allows you to release physically provisioned space which is associated with logically free space on ECKD volumes.

Space Release Enhancements for Global Copy and Global Mirror

With DS8000 Release 8.5, the Space Release functionality is now supported for Global Copy and Global Mirror primary and secondary volumes.

Unallocated space can now be released on active Global Copy and Global Mirror primary volumes.  The DS8880 Storage System will then also release the corresponding space on the secondary volumes, including volumes which are in Cascaded or Multiple Target PPRC configurations.

It is not required that the primary and secondary volumes have the same extent sizes.  The DS8000 will determine the correct extents which are eligible to be released even when different extent sizes are used.

Because Global Copy and Global Mirror are asynchronous replication functions, the data on the secondary volumes may not be as current as the data on the primary volumes.  The DS8000 ensures that the release space operations on the secondary volume are performed at the appropriate time to ensure the correct ordering of data.

Consider the following topology with Global Mirror:

When a SPACEREL command is given for a production volume at H1, the DS8000 records the extents which are eligible to be released.  After a Global Mirror consistency group is formed, the DS8000 releases the unused extents on the primary volume and informs the secondary volume of the released extents.  At the appropriate time, when the extents are no longer required at the remote location for any prior consistency group, the DS8000 releases the space on the secondary volume.  This all happens asynchronously to the SPACEREL command itself.

Metro Global Mirror

In a Multiple Target PPRC topology, space is released on the primary volume H1 and also on the target volumes, H2 and H3.

Cascaded topologies are also supported.  After releasing space on the primary production volume H1, the DS8000 sends commands to H2 to inform it of the space which can be released.  H2 in turn sends commands to H3 to also inform H3 of the space which can be released space.  The actual releasing of the physical storage is performed at the appropriate time for each type of replication.

In both Multiple Target PPRC and Cascaded PPRC topologies, space is released on the target H2 and H3 volumes by the DS8000s without any further action by the user.

Continuing Enhancements

These additions to the Space Release capability for Global Copy and Global Mirror that were delivered in Release 8.5 are one additional step in IBM’s continuing enhancements to the Extent Space Efficiency functionality in the DS8000 Storage System.

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