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Don Zoomik
Asked: 2019-10-01 11:31:53 +0800 CST

How to detect VMware snapshot presence from within VM?

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I want to perform Linux fstrim or Windows degrag retrim on a file system within VM to shrink VMware vSphere thin disks. However this should be performed only if there is no snapshot present as fstrim-ing a snapshoted file system will cause it to grow to full size on VMware vSphere thin disks.

Limitation: VM guest OS does not have access to vSphere/vCenter management interfaces.

Is there a way to detect presence of snapshot from within VM guest OS - for example via via VMware tools?

vmware-esxi

1 Answers

  1. Q: Is there a way to detect presence of snapshot from within VM guest OS*

    A: not that I know. However, you can use the sg_vpd to check if UNMAP is supported. For example:

    # sg_vpd /dev/sdb -p lbpv
Logical block provisioning VPD page (SBC):
 Unmap command supported (LBPU): 1
 Write same (16) with unmap bit supported (LBWS): 0
 Write same (10) with unmap bit supported (LBWS10): 0
 Logical block provisioning read zeros (LBPRZ): 1
 Anchored LBAs supported (ANC_SUP): 0
 Threshold exponent: 1
 Descriptor present (DP): 0
 Provisioning type: 2

    Some explanations:

    • provision type is 2, so you are running from a "thin" disk;
    • unmap command supported (LBPU) is 1, so your disk claims to support UNMAP/TRIM

    That said, issuing fstrim on a snapshotted disk should not have any adverse effect. On VMWare <= 6.5, it should have no effect at all, while VMWare >= 6.7 seems to support TRIM even when using snapshots.

    EDIT: as detailed in the comments below, the problem is related to original disk inflation during/after "trimmed" snapshot consolidation. As shown here, this side effect seems not avoidable even on latest VMWare 6.7

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