vSphere 6.5 UNMAP patch

A quick FYI for you about a VMware patch. VMware has released vSphere 6.5 p01 which, among other things, fixes a particular issue related to automated Guest OS UNMAP (does not impact automated UNMAP on datastores). The particular bug fix of note which can be seen in the KB is:

  • Tools in guest operating system might send unmap requests that are not aligned to the VMFS unmap granularity. Such requests are not passed to the storage array for space reclamation. In result, you might not be able to free space on the storage array.

The issue only impacts certain configurations. For instance, Windows environments which use a 4KB allocation unit on their NTFS drives will be unable to reclaim the Guest OS space because of the misalignment. For Linux, if you cannot use the discard option but have to manually use fstrim, you also cannot reclaim Guest OS space. Cormac Hogan of VMware explains it here in the following post. Remember that to use Guest OS UNMAP there are pre-requisites. You can find those in my previous post.

Invariably the question is should I apply the patch? Well you’ll see plenty of posts/information out there that says, “yes, immediately.” If the patch was non-disruptive I’d agree but it does require a reboot of the host. If you aren’t using Guest OS UNMAP, there is no urgency to apply the patch. If you are using it, but are diligent in using a 64KB AU and the discard option in Linux, well you probably can also wait. So I would say if you can apply it (multiple hosts so you can avoid downtime), it is always good to be at the latest release, but I think your use of Guest OS UNMAP is the determining factor here.



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