Reclaiming Space on a Thin-Provisioned VMFS Volume

The benefits of thin provisioning and just-in-time provisioning are well understood. A large number of our customers use thin provisioned LUNs for their VMware ESXi deployments. Occasionally I get questions like – “I just deleted some VMs on my thin provisioned LUN, but the free space does not show up on my Tegile array dashboard. How come? Does the Tegile array support VAAI UNMAP?”

Of course, Tegile Flash Storage Arrays support VAAI UNMAP to reclaim space on thin provisioned storage devices. The reason that the Tegile arrays do not show the free space is due to the fact that VMware does not reclaim space automatically. You need to manually run an unmap command on the ESX shell. Here is the shell command for ESXi 5.5:

# esxcli storage vmfs unmap -l

For ESXi 5.0 and 5.1, run the following commands:

# cd /vmfs/volumes/
# vmkfstools –y <% of the space to reclaim>

To understand why VMware does not do automatic space reclamation, please see Cormac Hogan’s blog post: vSphere 5.5 Storage Enhancements Part 4: UNMAP

Space reclamation takes up significant resources and can cause a slow down in overall performance. To get a sense of the performance impact of VAAI UNMAP and space reclamation, on an ESXi 5.5 host, I migrated (SVMotion) a 20GB virtual machine from the local disk to an FC LUN on a Tegile array; I then migrated the VM from the FC LUN to another local disk; lastly, I ran the unmap command on the ESX shell. Below is a screen shot that shows the relative I/O numbers.

Tegile VAAI UNMAP Diagram

In this case the ESX space reclamation operation generated around 1600 IOPS with 40% reads and 60% writes. To put this number in perspective, given that a moderate VDI workload generates anywhere between 10 to 100 IOPS per desktop, the IO generated by space reclamation is roughly equivalent to the load generated by 16 to 160 additional desktops. During peak production hours, this additional load could be significant for traditional storage arrays.

With the very high IOPS available from Tegile storage arrays, customers can size and deploy the storage to easily handle the UNMAP I/O bursts.

To learn more about how Tegile flash storage arrays support VMware, download our best practices guide:

VMware vSphere Best Practices Guide

Tegile Flash Storage & VMware vSphere Best Practices Guide

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