Aug
For those of us who have been excited about the new features in XenServer 5.5 there are definitely some new caveats to struggle through.
One of the exciting new features is snapshotting of virtual machines along with cloning those snaphots quickly into virtual machines. one of the downsides of this feature is the storage requirements. As of now the snapshots are written to the same volume as the virtual machine it is being snapped from.
Pay attention to the volume sizes, and snapshot sizes in the narrative below
VM size: 24GB
Pre-vm storage : 631.1GB used/1156.6GB total
VM installed
pre-snap: 655.1GB used/1156.6GB total 645.1GB allocated
snapshot of 24GB VM taken
snapsize 75.6MB according to snapshot tab
post-snap: 679.1GB used/1156.6GB total 645.1GB allocated
So based on this when you snap it is virtually nothing size-wise according to the snapshot tab and total allocated, but your used space does take up the full vm size so that you can’t over allocate. This is the error we got when snapping.
based on this We will need double the vm size to take the snap then.
VM created from snapshot
Post-deploy from snap: 703.1GB used/1156.6GB total 669.1GB allocated
In addtion
If I delete the snapshot my used space doesn’t change at all, the space isn’t reclaimed. But if I retake a snapshot my used space does not go up. If you want to be technical it went from 703.1GB to 703.2GB.
Not sure if this is a bug or a feature but it is clear that the snapshotting in XenServer is clearly in its infancy stage.
Next up:
Some strategies for working around this.
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Excellent article. I was very excited to see snapshot capability arrive with Xenserver 5.5, but was very disappointed to find:
(1) when one upgrades to 5.5 you dont get the same snapshot efficiency as if you create a new SR on the 5.5 system. There is no migration tool for this and for us + others i talk to this is a mess as we have TB or VMS that we cannot just shutdown and “copy over” to a new SR.
(2) Performance of VHD over LVM (LVHD) is supposed to be quite poor from what i have read vs native LVM. We would be interested in a comparison between Vmware filesystem + snapshot compared to Xenserver 5.5 both ease of use, disk space requirements and performance penalties.
(3) I do know in Xenserver Essentials 5.5 Citrix wants you to leverage the snapshot + cloning capabilites of the SAN, but we dont all run the limited SAN list that they are compatible with.
(4) VVI (Symantecs Frankenstein of Xenserver + Veritas filesystem) is supposted to be the best of both worlds with all sorts of fancy file system features integrated with Xenserver including non-stop migration of VMs between storage repositories etc.
comments welcome – keep up the good work – i look forward to “part II” of your article above.
rgds,
Eric -
Secure Technology Group LLC.
I have a question that you guys can probably answer. Our SAN sales rep is also a VMWare guru. We chose XenServer to be our VMhost. I started snapshotting servers after the P2V process and now I have all this space that I wanted to free up. According to our Rep, if you delete a VMWare rep, you can screw up your original and he doesn’t recomend snaps at all. He thought Xen could hose up as well. I don’t understand this as he says the snaps are being written to as well as the actual running VM. That doesn’t make sense to me, why make a snap if it is still going to be updated? Anyway, can I delete my snaps without hurting the original? Also, any answer to freeing the disk space after doing so?
Thanks,
Ernie
For the record, Citrix just posted a new update to XenServer (5.5. update 1), which allows you to manually reclaim the ‘lost’ storage space that you don’t get back when you delete a snapshot. You can check this out here
Its kind of a pain to remember to run a script manually every time you trigger a snapshot, but there are already articles and tools coming out to ease this burden, like QuorumSoft’s Alike, which can schedule and automate the process.
Also for the record, Citrix does have a roadmap for this issue, and they expect to release a more automated solution to this problem in Q12010 or so, but seeing how long it took them to react to this problem, I’m not holding my breath for their solution to be too timely.
-AHS
For the record, Citrix just posted a new update to XenServer (5.5. update 1), which allows you to manually reclaim the ‘lost’ storage space that you don’t get back when you delete a snapshot. You can check this out here
Its kind of a pain to remember to run a script manually every time you trigger a snapshot, but there are already articles and tools coming out to ease this burden, like QuorumSoft’s Alike, which can schedule and automate the process.
Also for the record, Citrix does have a roadmap for this issue, and they expect to release a more automated solution to this problem in Q12010 or so, but seeing how long it took them to react to this problem, I’m not holding my breath for their solution to be too timely.
-AHS