Using Microsoft Windows Server 2012 thin provisioning space reclamation on IBM XIV

Earlier last month the IBM XIV 11.2 microcode was announced.  One of the features for this code is support for the T10 SCSI UNMAP standard.  The benefit of SCSI UNMAP is that it solves a problem commonly associated with thin provisioned volumes.

Typical behavior of thin provisioned volumes is that as data is generated and space consumed, the thin provisioned volume in turn consumes more physical capacity.  If data is removed from the thin provisioned volume (for example a virtual machine migration) that space is freed on the file system, however the physical capacity on the storage system remains consumed.  This puts your physical storage capacity in a state of perpetual growth.  There have been solutions (usually manual), that consisted of running tools like SDelete and then mirroring the volume.

Enter SCSI UNMAP which solves this problem by issuing SCSI commands to the physical storage when blocks are freed.  In simple functional terms, if I free up space on my file system by for example migrating a virtual machine, then that space would also be reclaimed from the physical capacity on the storage system.  More details on SCSI UNMAP is available in this Microsoft document.

SCSI UNMAP is a standard so the support built into XIV should enable other applications to take advantage of it in the future.  For example VMware released SCSI UNMAP before announcing issues, eventually this functionality should be available.

Microsoft Windows Server 2012 included thin provisioning support of SCSI UNMAP upon its release and is the key application supported by XIV in the 11.2 code.  IBM has released a comprehensive white paper which discusses this feature in greater detail and also discusses implementation considerations.  The white paper is a good read for anyone but particularly those that are running or will be running Windows Server 2012 on IBM XIV.

Microsoft Windows Server 2012 thin provisioning space reclamation using the IBM XIV Storage System

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