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

[Dietmar Maurer]

> > 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.

If there are many writes from the guest, you still get many 64KB block.

Anyways, and additional RPC layer always adds overhead, and It can be 
completely avoided.
Maybe not much, but I want to make backup as efficient as possible.