Re: Cache modes libvirt

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Some information is missing to give a helpful answer.

How do you backup? (Files? RBD via Ceph? Block Device with qemu-img? Which
Device driver do you use (Virtio? SATA?).

In our production we use Virtio RBD and the Hypervisor standard cache mode.
The Disks are snapshoted before the backup with 'qemu-img', e. g.:
virsh snapshot-create-as VM backup-VM --diskspec
vda,file=/snapshots/backup-snapshot-VM-vda.raw --disk-only --atomic
--quiesce --no-metadata
qemu-img convert -O raw rbd:libvirt-pool/VM.raw /backup/VM.raw -p
virsh blockcommit VM vda --wait --active --pivot

The backup process is as fast as expected.

Am Mi., 30. Nov. 2022 um 16:09 Uhr schrieb Dominique Ramaekers <
dominique.ramaekers@xxxxxxxxxx>:
>
> Hi,
>
> I was wondering...
>
> In Ceph/Libvirt docs only cachmodes writetrough and writeback are
discussed. My clients's disks are all set to writeback in the libvirt
client xml-definition.
>
> For a backup operation, I notice a severe lag on one of my VM's. Such a
backup operation that takes 1 to 2 hours (on a same machine but) on local
LVM storage, takes on a ceph storage 6 hours. Though I have to say that the
cachemode on the local LVM storage is set on directsync.
>
> => So what if I played around with other cachemodes? Will it make a
difference?
>
> I'm trying now cachemode 'unsafe' so tonight I should measure the
difference if there is any.
>
> Input will be greatly appreciated.
>
> Greetings,
>
> Dominique.
>
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