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


  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.

vSphere 6.5 guest UNMAP may cause VM I/O latency spikes – fixed in update 1

I converted some VMs to thin and upgraded VM hardware version to 13 to test out savings. Initial retrim caused transient I/O slowdown in VM but the issue kept reappearing randomly. I/O latency just spikes to 400ms for minutes for no apparent reason. It also seems to affect other surrounding VMs, just not as badly. After several days, I converted VMs back to thick and issues disappeared.

I’m not sure where the problem is and I can’t look into it anymore. Might be a bug in vSphere. Might be the IBM v7000 G2 SAN that goes crazy. As I said, I cannot investigate it any further but I’ll update the post if I ever hear anything.

PS! Savings were great, on some systems nearly 100% from VMFS perspective. On some larger VMs with possible alignment issues, reclamation takes several days though. For example, a 9TB thick file server took 3 days to shrink to 5TB.

Update 2017.o6.29:

Veeam’s (or Anton Gostev’s) newsletter mentioned a similar issue just as I came across this issue again in a new vSphere cluster. In the end VMware support confirmed the issue with expected release of 6.5 Update 1 at the end of July.

Update much later in november

I’ve been running Update 1 since pretty much release date and UNMAP works great! No particular performance hit. Sure, it might be a bit slower during UNMAP run but it’s basically invisible for most workloads.

I’ve noticed that for some VM’s, you don’t space back immediately. On some more internally fragmented huge (multi-TB) VMs, particularly those with 4K clusters, space usage seems to reduce slowly over days or weeks. I’m not sure what’s going on but perhaps ESXi is doing some kind of defrag operation in VMDK…? And yeah, doing a defrag (you can do it manually form command line in Windows 2012+) and then UNMAP helps too.

vSphere 6.5 virtual NVMe does not support TRIM/UNMAP/Deallocate

I was playing with guest TRIM/UNMAP the other day and looked at new shiny virtual NVMe controller. While it would not help much in my workloads, cutting overhead never hurts. So I tried to do “defrag /L” in VM and it return that device doesn’t support it.

So I looked up release notes. Virtual NVMe device: “Supports NVMe Specification v1.0e mandatory admin and I/O commands”.

The thing is that NVMe part that deals with Deallocate (ATA TRIM/SCSI UNMAP in NVMe-speak) is optional. So back to pvscsi for space savings…