Re: Using blockdev-mirror for Live Backup

[Ali Gholami Rudi]

Alberto Garcia <berto@igalia.com> wrote:
> On Tue, May 16, 2023 at 11:58:30PM +0330, aliqrudi@gmail.com wrote:
> 
> > We are executing hundreds of VMs in our hosts (dual AMD EPYC 7002
> > CPUs + 2TB memory + 8 NVMe disks).  To cope with disk failures, we
> > tried different Linux software RAID mechanisms.  At high disk usage,
> > all of them faced unexpectedly high CPU load, mostly in the kernel.
> > Therefore we decided not to use software RAID.
> 
> An alternative is the Quorum block driver:
> 
>    -drive if=virtio,driver=quorum,
>           children.0.file.filename=disk0.qcow2,
>           children.1.file.filename=disk1.qcow2,
>           children.2.file.filename=disk2.qcow2,
>           vote-threshold=2
> 
>    (this is supposed to go all in one line)
> 
> This is similar to RAID-1, but note that the disk images themselves
> do not contain any kind of additional metadata and they don't know
> that they are part of a Quorum array, it's your responsibility to put
> identical images together.

Thanks for your answer.

I have just tested them and blockdev-mirror seems much faster for disk
synchronization than quorum arrays.

Is there any reason that the incremental sync mode is not implemented
for blockdev-mirror?  At the moment there are full and none sync modes:
- Full mode synchronizes the whole disk.
- None mode synchronizes new writes.

The problem is that, without persistent dirty bitmaps, it does not
seem possible to synchronize the disk effciently, if the mirror has
been (maybe partially) synchronized before.

If I prepare a patch, do you think it would be merged?

        Ali