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.

I just reread the code and noticed the delay is 1 microsecond, not 1
millisecond.

The problem is that this is a magic value.  It worked on your machine
with your workload, but there's no guarantee it works on anyone else's
machine or workload.  We can't depend on magic values like this.

If you want to implement an alternative to rate-limiting, please do it
in a separate patch series and make it work for all block job types.

Stefan