Hyper-V UNMAP Does Work With SAN Snapshots And Checkpoints But Not Always As You First Expect

Recently I was asked to take a look at why UNMAP was not working predictably  in a Windows Server 2012 R2 Hyper-V environment. No, this is not a horror story about bugs or bad storage solutions. Fortunately, once the horror option was of the table I had a pretty good idea what might be the cause.

San snapshots are in play

As it turned out everything was indeed working just fine. The unexpected behavior that made it seem that UNMAP wasn’t working well or at least at moments they didn’t expected it was caused by the SAN snapshots. Once you know how this works you’ll find that UNMAP does indeed work predictably.

Snapshots on SANs are used for automatic data tiering, data protection and various other use cases. As long as those snapshots live, and as such the data in them, UNMAP/Trim will not free up space on the SAN with thinly provisioned LUNs. This is logical, as the data is still stored on the SAN for those snapshots, hard deleting it form the VM or host has no impact on the storage the SAN uses until those snapshots are deleted or expire. Only what happens in the active portion is directly impacted.

An example

  • Take a VM with a dynamically expanding VHDX that’s empty and mapped to drive letter D. Note the file size of the VHDX and the space consumed on the thinly provisioned SAN LUN where it resides.
  • Create 30GB of data in that dynamically expanding  virtual hard disk of the virtual machine
  • Create a SAN snapshot
  • Shift + Delete that 30GB of data from the dynamically expanding virtual hard disk in the virtual machine. Watch the dynamically expanding VHDX  grow in size, just like the space consumed on the SAN
  • Run Optimize-Volume D –retrim to force UNMAP and watch the space consumed of the Size of the LUN on the SAN: it remains +/- the same.
  • Shut down the VM and look at the size of the dynamic VHDX file. It shrinks to the size before you copied the data into it.
  • Boot the VM again and copy 30GB of data to the dynamically expanding VHDX in the VM again.
  • See the size of the VHDX grow and notice that the space consumed on the SAN for that LUN goes up as well.
  • Shift + Delete that 30GB of data from the dynamically expanding  virtual hard disk in the virtual machine
  • Run Optimize-Volume D –retrim to force UNMAP and watch the space consumed of the Size of the LUN on the SAN: It drops, as the data you delete is in the active part of your LUN (the second 30GB you copied), but it will not drop any more than this as the data kept safe in the frozen snapshot of the LUN is remains there (the first 30GB you copied)
  • When you expire/delete that snapshot on the SAN  we’ll see the size on the thinly provisioned SAN LUN  drop to the initial size of this exercise.

I hope this example gave you some insights into the behavior


So people who have snapshot based automatic data tiering, data protection etc. active in their Hyper-V environment and don’t see any results at all should check those snapshot schedules & live times. When you take them into consideration you’ll see that UNMAP does work predictably, all be it in a “delayed” fashion Smile.

The same goes for Hyper-V checkpoints (formerly known as snapshots). When you create a checkpoint the VHDX is kept and you are writing to a avhdx (differencing disk) meaning that any UNMAP activity will only reflect on data in the active avhdx file and not in the “frozen” parent file.

TRIM/UNMAP Support in Windows Server 2012 & Hyper-V/VHDX


I’m very exited about the TRIM/UNMAP support in Windows Server 2012 & Hyper-V with the VHDX file. Thin provisioning is a great technology. It’s there is more to it than just proactive provisioning ahead of time. It also provides a way to make sure storage allocation stays thin by reclaiming freed up space form a LUN. Until now this required either the use of sdelete on windows or dd for the Linux crowd, or some disk defrag product like Raxco’s PerfectDisk. It’s interesting to note here that sdelete relies on the defrag APIs in Windows and you can see how a defragmentation tool can pull off the same stunt. Take a look at Zero-fill Free Space and Thin-Provisioned Disks & Thin-Provisioned Environments for more information on this. Sometimes an agent is provided by the SAN vendor that takes care of this for you (Compellent) and I think NetApp even has plans to support it via a future ONTAP PowerShell toolkit for NTFS partitions inside the VHD (https://communities.netapp.com/community/netapp-blogs/msenviro/blog/2011/09/22/getting-ready-for-windows-server-8-part-i).  Some cluster file system vendors like Veritas (symantec) also offer this functionality.

A common “issue” people have with sdelete or the like is that is rather slow, rather resource intensive and it’s not automated unless you have scheduled tasks running on all your hosts to take care of that. Sdelete has some other issue when you have mount points, sdelete can’t handle that. A trick is to use the now somewhat ancient SUBST command to assign a drive letter to the path of the mount point you can use sdelete. Another trick would be to script it yourself see. Mind you can’t just create a big file in a script and delete it. That’s the same as deleting “normal” data and won’t do a thing for thing provisioning space reclamation. You really have to zero the space out. See (A PowerShell Alternative to SDelete) for more information on this. The script also deals with another annoying thing of sdelete is that is doesn’t leave any free space and thereby potentially endangers your operations or at least sets off all alarms on the monitoring tools. With a home grown script you can force a free percentage to remain untouched.


With Windows Server 2012 and Hyper-V VHDX we get what is described in the documentation  “’Efficiency in representing data (also known as “trim”), which results in smaller file size and allows the underlying physical storage device to reclaim unused space. (Trim requires physical disks directly attached to a virtual machine or SCSI disks in the VM, and trim-compatible hardware.)  It also requires Windows 2012 on hosts & guests.

I was confused as to whether VHDX supports TRIM or UNMAP. TRIM is the specification for this functionality by Technical Committee T13, that handles all standards for ATA interfaces. UNMAP is the Technical Committee T10 specification for this and is the full equivalent of TRIM but for SCSI disks. UNMAP is used to remove physical blocks from the storage allocation in thinly provisioned Storage Area Networks. My understanding is that is what is used on the physical storage depends on what storage it is (SSD/SAS/SATA/NL-SAS or SAN with one or all or the above) and for a VHDX it’s UNMAP (SCSI standard)

Basically VHDX disks report themselves as being “thin provision capable”. That means that any deletes as well as defrag operation in the guests will send down “unmaps” to the VHDX file, which will be used to ensure that block allocations within the VHDX file is freed up for subsequent allocations as well as the same requests are forwarded to the physical hardware which can reuse it for it’s thin provisioning purpose. Also see http://msdn.microsoft.com/en-us/library/hh848053(v=vs.85).aspx

So unmap makes it way down the stack from the guest Windows Server 2012 Operating system, the VHDX , the hyper visor and the storage array.This means that an VHDX will only consume storage for really stored data & not for the entire size of the VHDX, even when it is a fixed one. You can see that not just the operating system but also the application/hypervisor that owns the file systems on which the VHDX lives needs to be TRIM/UNMAP aware to pull this off.

The good news here is that there is no more sdelete to run, scripts to write, or agents to install. It happens “automagically” and as ease of use is very important I for one welcome this!  By the way some SANs also provide the means to shrink LUNs which can be useful if you want the space used by a volume is so much lower than what is visible/available in windows and you don’t want people to think you’re wasting space or all that extra space is freely available to them.

To conclude I’ll be looking forward to playing around with this and I hope to blog on our experiences with this later in the year. Until Windows Server 2012 & VHDX specifications are RTM and fully public we are working on some assumptions. If you want to read up on the VHDX format you can download the specs here. It looks pretty feature complete.