Whenever you create a snapshot
disk it will increase the length of the “snapshot chain” by 1 and
there is a hard length limit of 30. However whenever you delete a snapshot disk
the background “coalesce” process should be able to reduce the “snapshot
chain” length again. So your procedure (create snapshot; export snapshot;
delete snapshot) ought to work fine. This is in fact the same process followed
by the “VM protection and recovery” feature which will be part of
The problem you linked to is
related but not quite the same: it refers to a limitation within the “coalesce”
process where it cannot reduce a chain of length 2 into a chain of length 1.
The only reason people worry about this is because a chain of length 2 can take
up a lot more space on LVM where there is no “thin provisioning” –
this isn’t a problem for you on NFS.
Given your observation that this
only happens to VMs with more than 1 attached virtual disk, I think you’ve
found a bug in the tool which you used to add the second disk to the VM. When
you delete a VM snapshot (or uninstall a VM or template generally), only those
disks which have a special flag set are actually deleted (specifically those
with VBD.other-config:owner IIRC). If you use the “xe vm-disk-add”
CLI command it should set this flag. If this flag isn’t set then it would
cause the additional disks not to be deleted… this would cause your “snapshot
chain” to get ever bigger, hit 30 and then fail.
I think a workaround is to
identify the disk (VDI not VM) snapshots which haven’t been deleted and
delete them manually, possibly via the CLI (“xe vdi-destroy”). After
a short while the background “coalesce” process should kick in and
reduce your chain lengths again.
[mailto:xen-api-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Simone Cariani
Sent: 12 November 2010 12:00
To: xen-api@xxxxxxxxxxxxxxxxxxx; xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-API] problem with snapshot unallocation
i use XCP 0.5 with a NFS storage repository and normally i
do daily snapshots from all VMs i have, as follow:
1) create snapshot
2) export snapshot to file
3) delete snapshot
One day I get the VM error "Snapshot chain is too
long". I notice however that this appears to only VM's with more than 1
attached virtual disk. In my case I have a couple VM's with 2 disks, and there
were many orphaned copies of only 1 of the 2 disks. >From OpenXenManager is
not possible to delete these snapshots..
I read a problem like this on Xencenter forum
and the solution is
Does someone know how to fix this bug on XCP 0.5 ?