Sunday, 23 October 2011

Experiment: Is it Possible to Shrink Back Down a Thin-Provisioned LUN in SAN/iQ 9.0?

Note 1: See Appendix below for more on VMware's vStorage APIs
Note 2: SAN/iQ 9.5 was released 7 October 2011 (see )

Prologue: A problem with thin-provisioned LUNs is that, when data is removed from inside a LUN, the LUN does not shrink down correspondingly (unless using really high-end storage like 3PAR.)

Question: SAN/iQ 9.0 supports VMware's vStorage APIs (VAAI) with vSphere 4 and above, so a quick experiment is devised to see if a thin-provisioned SAN/iQ 9.0 LUN, when grown by adding data (a storage vMotion,) and then having the data deleted, and on converting the volume from thin to thick and then thick to thin, does it shrink back down?

Answer: No (the expected result)

Experimental Results

1: Start with unpopulated thin-provisioned volume →
TEST LUN created of size 25GB

2: Create a datastore →
Datastore details below (this is VMFS 3.46 and notice Hardware Acceleration = Supported)

3: Populate the datastore with data (here a small XP VM was storage vMotioned) →
TEST LUN has grown to 11.92GB consumed space

4: Then delete the data from the datastore
TEST LUN consumed space remains at 11.92GB

5: Convert the LUN to full provisioned
TEST LUN consuming 25GB space now (says only 13.09GB reclaimable)

6: Convert the LUN back to thin
TEST LUN still consuming 11.92GB space

Appendix: VMware's vStorage APIs

Full Copy: This feature delivers hardware-accelerated copying of data by performing all duplication and migration operations on the array. Customers can achieve considerably faster data movement via VMware Storage vMotion, and virtual machine creation and deployment from templates and virtual machine cloning.

Block Zero: This feature delivers hardware-accelerated zero initialization, greatly reducing common input/output tasks such as creating new virtual machines. This feature is especially beneficial when creating fault-tolerant (FT)-enabled virtual machines or when performing routine application-level Block Zeroing.

Hardware-assisted locking: This feature delivers improved locking controls on Virtual Machine File System (VMFS), subsequently allowing far more virtual machines per datastore and shortened simultaneous block virtual machine boot times. This improves performance of commons tasks such as virtual machine migration, powering many virtual machines on or off and creating a virtual machine from template.

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