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XenServer 5.5 snapshots and storage consumption

October 19, 2009 Leave a comment Go to comments

I’m sure most of you used the Snapshot function provided with XenServer 5.5 and wondered why the storage is still allocated after deleting the snapshot of a VM?
Same issue occurs after cloning a VM (Fast cloning) and delete the clone. This is indeed a problem, but not a “bug” as Citrix confirmed and it depends on the SR you use.

There is a very interesting thread “Missing free space after making a snapshot and deleting that snapshot” on the Citrix forum. People are very frustrated and call for a “hotfix”.
Citrix confessed the problem and soon there will be a “fix” or update for it, remember not a “hotfix”, because Citrix says this behaviour by design.

The more fundamental issues are being addressed as follows:
1. Short term:
Issue: VMs lose sparse-awareness during full copy and export operations. This prevents base-copy VDIs from being deflated and consuming less space.
Fix: Hotfix being created for this issue, specifically with XAPI.
Timeframe: XenServer 5.5 Update 1, late November or early December
2. Medium term
Issue: After all snapshots are deleted in XenCenter, base-copy VDIs are not coalesced
Proposed fix: Create a tool that can coalesce VDIs on suspended or powered down VMs, and remove the base-copy VDIs to reclaim all the space.
Timeframe: Hotfix being created and tested now, may be included in XenServer 5.5 Update 1. If not, it will be provided as a separate hotfix afterwards.
3. Medium to Long term
Issue: Same as #2
Proposed fix: Have XenServer automatically coalesce VDIs on live VMs automatically without administrative intervention.
Timeframe: A future XenServer release, possibly as early as H1 2010.

-Bill Carovano from Citrix-


Meanwhile Citrix released a Document called XenServer: Understanding Snapshots.
This document describes the Snapshot function in detail, what a bummer, i think many people wish this document would have been released earlier…

Categories: XenServer
  1. justanothersysadmin
    January 7, 2010 at 03:06

    We found a bit of a ‘workaround’ to the sparse-awareness issue. We were trying to get a bunch of blank disks copied on a thin-provisioned NetApp SR and ran into a bunch of the symptoms you described above.

    By fluke, we found on our SR that if we created a new machine from a VM template (not just a copy, such as xe vm-copy, but creation from a new template, i.e. xe vm-install), ‘sparseness’ was in tact on the provisioned VDI’s.

    So, basically, although we just needed a series of VDI’s from a template disk, we ended up doing the following in order to get the thin provisioning of the disks to occur:

    1) Create a new VM with right size of disk.
    2) Don’t install an OS or anything on that VM.
    3) Detach from that VM the VDI that was provisioned for it.
    4) Attach the VDI to a working VM.
    5) Do whatever operations need to be done to the VDI disk (partitioning, formatting, adding data, etc).
    6) Detach the VDI from the working VM and re-attach it to the new VM that was created in step 1.
    7) Convert the VM into a template.
    8) Create new VM’s from that template either through XenCenter (slow) or using xe commands (scripted it to make 34 VM’s/disks for us ;)).
    9) Kill the VM’s that were created, but leave the VDI’s in tact.

    TADA! Thin-provisioned VDI’s!!! (YMMV).

    I am not sure what mix of software etc. resulted in this issue, but we are currently running XS5.5 (Essentials) with storage provided by a NetApp FAS3140 running Data ONTAP 7.3.2, and this workaround worked for us.

  1. November 20, 2009 at 12:08

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