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Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Int

From: Kevin Wolf
Subject: Re: [Qemu-devel] [RFC] block-trace Low Level Command Supporting Disk Introspection
Date: Wed, 15 May 2013 11:46:02 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Am 15.05.2013 um 11:16 hat Paolo Bonzini geschrieben:
> > > > 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.

On second thought, can't you do zero copy anyway for full cluster
writes? This means that at most two clusters per request must be
allocated, no matter how large it is, and you can probably reuse the
same one-cluster buffer for both.

> > 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).

Yes, that sounds like a good use case.

But does this really cover all use cases a real synchronous active
mirror would provide? I understood that Wolf wants to get every single
guest request exposed e.g. on an NBD connection.


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