[Qemu-devel] Re: [PATCH 2/3 v5] Block live migration

[Liran Schour]

Jan Kiszka <address@hidden> wrote on 26/11/2009 15:53:49:


> > +            qemu_get_buffer(f, buf,
> > +                            BLOCK_SIZE);
> > +            if(bs != NULL) {
> > +
> > +                bdrv_write(bs, (addr >> SECTOR_BITS),
> > +                           buf, block_mig_state->sectors_per_block);
>
> This synchronous write-back translates appears to be the reason for an
> unusable migration (or restore from snapshot) speed: about 100 KB/s here
> (ie. 22h for a rather small 8G guest :( ). Did you already try to
> improve this situation?

I have seen this behavior, but it seems that there is a very big difference
in the performance if the new block device is based on an allocated file
already (try the same migration to an already allocated file in the
requested size) I am trying to figure out why we see this behavior.(any
ideas?)
Anyway we can turn the writes to be async but we have to synchronize all
destinations writes before completing the migration and moving the guest to
the destination. When the guest starts to run on the destination all writes
should be finished, so anyhow we need to wait synchronously to the writes.
I will look on this further next week.

- Liran