Running UNMAP on snapshotted VMware hardware 11+ thin VMs may cause them to inflate to full size

Scenario:

  1. ESXi 6.X (6.5 in my case)
  2. VMware hardware 11+ (13)
  3. Thin VM
  4. UNMAP aware OS (Windows 2012+)

If you snapshot VM and run UNMAP (for example retrim from defrag utility), VM may (not always) inflate to full size during snapshot commit. It also results in really long commit times.

I’ve seen it happen quite a few times and it’s really annoying if you for some historical reason have tons of free space on drives (for example NTFS dedup was enabled long after deployment) and may even cause datastores to become full (needless to say, really bad). Also it tends to happen during backup windows that keep snapshots open for quite a while (usually at night, terrible). While I could disable automatic retrim (bad with lots of small file operations, normal UNMAP isn’t very effective on them due to alignment issues) or UNMAP (even worse), it’s an acceptable risk for now if you keep enough free space on datastore to absorb inflation of the biggest VM. You can retrim after snapshot commit and it drops down to normal size quickly (minutes).

I haven’t seen this anywhere else but I guess I’ll do a reproducable PoC, contact VMware support and do an update.

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