Re: [Qemu-devel] [PATCH v4 2/6] add basic backup support to block driver

[Stefan Hajnoczi]

On Mon, Feb 25, 2013 at 03:49:11PM +0000, Dietmar Maurer wrote:
> > > We previously used LVM and run backup with 'idle' IO priority (CFQ) to 
> > > avoid
> > such behavior.
> > >
> > > But qemu does not provide an IO queue where we can set scheduling
> > priorities?
> > 
> > QEMU block jobs support rate-limiting.  Set it to 10-20% of the disk's 
> > throughput
> > and the slowness should go away but the backup takes longer.
> 
> Sorry, but I don't want to set a rate limit. I want to set an IO priority - 
> this is something different.
> My solution does not delay the backup.

QEMU is not in a position to implement global scheduling (priorities).
The QEMU process only knows about one guest.  If you have multiple
guests on the host an I/O "priority" inside QEMU process A will not take
into account QEMU process B.

Your solution *does* delay backup by 1 ms each iteration :).

Stefan