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From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting
 Disk Introspection
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To: Kevin Wolf <kwolf@redhat.com>
Cc: Wolfgang Richter <wolf@cs.cmu.edu>, qemu-devel <qemu-devel@nongnu.org>, stefanha <stefanha@redhat.com>


> > We don't need a way to pass data from before to after hooks, a simple
> > scan of a linked list will do.
> 
> So in this case the linked list is the way.

Point taken. :)

> > > Does the "if there was no room" part mean that the mirror is active only
> > > sometimes?
> > 
> > Yes, otherwise the guest can allocate arbitrary amounts of memory in the
> > host just by starting a few very large I/O operations.
> 
> I think I would rather throttle I/O in this case, i.e. requests wait
> until they can get the space. At least for a synchronous mirror we
> have to do something like this.

Yes, but this is still asynchronous.  The active part is just an optimization
to avoid write amplification (where small random writes require I/O of an entire
block as big as the bitmap granularity).

> > > And why even bother with a dirty bitmap for an active mirror? The
> > > background job that sequentially processes the whole image only needs a
> > > counter, no bitmap.
> > 
> > That's not enough for the case when the host crashes and you have to
> > restart the mirroring or complete it offline.
> 
> You're thinking of a persistent bitmap here? Makes sense then, I didn't
> think about that.

Yes.

Paolo