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Date: Thu, 30 Jun 2011 15:38:29 -0300
From: Marcelo Tosatti <mtosatti@redhat.com>
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Subject: Re: [Qemu-devel] KVM call agenda for June 28
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To: Kevin Wolf <kwolf@redhat.com>
Cc: Chris Wright <chrisw@redhat.com>, KVM devel mailing list <kvm@vger.kernel.org>, quintela@redhat.com, Stefan Hajnoczi <stefanha@gmail.com>, Dor Laor <dlaor@redhat.com>, qemu-devel@nongnu.org, Avi Kivity <avi@redhat.com>

On Thu, Jun 30, 2011 at 04:52:00PM +0200, Kevin Wolf wrote:
> Am 30.06.2011 16:36, schrieb Marcelo Tosatti:
> >> 4. Live block copy API and high-level control - the main code that
> >> adds the live block copy feature.  Existing patches by Marcelo, can be
> >> restructured to use common core by Marcelo.
> > 
> > Can use your proposed block_stream interface, with a "block_switch"
> > command on top, so:
> > 
> > 1) management creates copy.img with backing file current.img, allows
> > access
> > 2) management issues "block_switch dev copy.img"
> > 3) management issues "block_stream dev base"
> 
> Isn't this block_switch command the same as the existing snapshot_blkdev?

Yep.

> > Thought of implementing "block_stream" command by reopening device with
> > 
> > blkstream:imagename.img
> > 
> > Then:
> > 
> > AIO_READ:
> > - for each cluster in request:
> >     - if allocated-or-in-final-base, read.
> >     - check write queue, if present wait on it, if not, add "copy"
> >       entry to write queue.
> >     - issue cluster sized read from source.
> >     - on completion:
> >         - copy data to original read buffer, complete it.
> >         - if not cancelled, write cluster to destination.
> > 
> > AIO_WRITE
> > for each cluster in request:
> >     - check write queue, cancel/wait for "copy" entry.
> >     - add "guest" entry to write queue.
> >     - issue write to destination.
> >     - on completion:
> >         - remove write queue entry.
> > 
> > 
> > With the 0...END background read, once it completes write final base
> > file for image.
> > 
> > So block_stream/block_stream_cancel/block_stream_status commands, the
> > background read and the rebase -u update can be separate from the block
> > driver.
> 
> The way how it works looks good to me, I'm just not entirely sure about
> the right place to implement it. I think request queueing and copy on
> read could be useful outside blkstream, too.

They could be lifted later, when there are other users.