Re: [Qemu-devel] [PATCH v4 3/6] add backup related monitor commands

[Stefan Hajnoczi]

On Wed, Mar 06, 2013 at 02:42:57PM +0000, Dietmar Maurer wrote:
> > > > Maybe you'd better use a different output format that doesn't
> > > > restrict you to 64k writes.
> > >
> > > The output format is not really the restriction. The problem is that
> > > an additional IPC layer add overhead, an d I do not want that (because it 
> > > is
> > totally unnecessary).
> > 
> > I missed the reason why you cannot increase the block size.  
> 
> When we run backup, we need to read such block on every write from the guest.
> So if we increase block size we get additional delays.

Don't increase the bitmap block size.

Just let the block job do larger reads.  This is the bulk of the I/O
workload.  You can use large reads here independently of the bitmap
block size.

That way guest writes still only have a 64 KB read overhead but you
reduce the overhead of doing so many 64 KB writes from the backup block
job.

Stefan