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Re: Live backups create-snapshot-as and memspec




El 25/2/2021 a las 11:37, Peter Krempa escribió:
On Wed, Feb 24, 2021 at 14:49:19 -0300, David Wells wrote:
Hi all!

I've been using libvirt for some time and until now I have treated backups
of virtual computers as if they where physical computers installing the
backup client on the guest. I am now however facing the need to backup a
couple a couple of guest at the host level so I've been trying to catch up
by reading, googling and by trial and error too. Up to now I've been able to
backup a live machine whith a command like the following

virsh snapshot-create-as --domain test --name backup --atomic --diskspec
vda,snapshot=external --disk-only
This command creates a file test.backup and in the meantime I can backup the
original test.qcow2 but for what I saw this disk image is in a "dirty"
state, as if the machine I could restore from this file had been turned off
whithout a proper shutdown.

I know that I can later restore the machine to its original state by issuing
a command like this

virsh blockcommit --domain test vda --active --pivot
virsh snapshot-delete test --metadata backup
I have seen that it is possible to create the snapshot using a memspec
parameter which would make the backup of the guest as if it where in a clean
state, however I haven't found the equivalent of the blockcommit for the
memory file, in a sort of speak, to be able to restore the guest to it's
original state.
The memory image file doesn't depend on any other state nor does the VM
use it after the snapshot is taken, if you don't need it you can delete
it.

The disk overlay you create by  'virsh snapshot-create-as' records only
differences to the original image, but for memory state it doesn't make
much sense as the memory is small and changes a lot, so we take a
snapshot of the whole memory.

Now to restore the state of a VM to a snapshot taken with memory,
libvirt's native APIs for reverting don't work on external snapshots
yet.

You can use 'virsh restore memimg' to load VM's state as the memory
snaphsot images have the same format as images created by 'virsh save'.

Please note though that the configuration of the VM is taken from the
save image if you do so, which includes paths to disk images, which may
no longer be correct or desired, but virsh save-image-edit can be used
to modify the XML if needed.

Hi all!

Peter, thank you very much for your reply! So if I understood correctly there is no way, at this time, to backup a live guest in a way that I can restore it to a "clean" state. Also, if I understood correctly, the probability of a corrupt system on restore of the vm is minimized having the qemu guest agent installed on the guest and issue the virsh create-snapshot-as with the --quiesce parameter, is this correct?

Thanks in advance!
Best regards,
David Wells.



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