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Date: Mon, 24 Feb 2014 17:26:28 -0500 (EST)
From: Paolo Bonzini <pbonzini@redhat.com>
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References: <52FDE495.4050004@ac.upc.edu> <52FDF6F0.4090405@redhat.com>
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Subject: Re: [Qemu-devel] [libvirt-users] Adjust disk image migration (NBD)
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Libvirt Users <libvirt-users@redhat.com>, Michal Privoznik <mprivozn@redhat.com>, Jeff Cody <jcody@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Joaquim Barrera <jbarrera@ac.upc.edu>

> Thanks for raising this.
> 
> I noticed that mirror_run() does not throttle the first loop where it
> populates the dirty bitmap using bdrv_is_allocated_above().

This is on purpose.  Does it causes a noticeable stall in the guest?

> The main
> copy loop does take the speed limit into account but perhaps that's
> broken too.

Yeah, it looks broken.  Each iteration of the loop can write much more
than sectors_per_chunk sectors, but here:

            if (s->common.speed) {
                delay_ns = ratelimit_calculate_delay(&s->limit, sectors_per_chunk);
            } else {
                delay_ns = 0;
            }

the second argument is fixed. :/

Paolo