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Date: Thu, 30 Jun 2011 16:52:00 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] KVM call agenda for June 28
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To: Marcelo Tosatti <mtosatti@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>

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?

> 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.

Kevin