Re: [Qemu-devel] [PATCH 3/3] Add nocreate option to snapshot_blkdev

[Federico Simoncelli <fsimonce@redhat.com>]

----- Original Message -----
> From: "Paolo Bonzini" <pbonzini@redhat.com>
> Cc: "Federico Simoncelli" <fsimonce@redhat.com>, kwolf@redhat.com, mtosatti@redhat.com, qemu-devel@nongnu.org
> Sent: Thursday, February 23, 2012 8:38:55 AM
> Subject: Re: [PATCH 3/3] Add nocreate option to snapshot_blkdev
> 
> On 02/23/2012 08:19 AM, Paolo Bonzini wrote:
> >> > Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
> > What is the usecase, and how can this be tested?
> 
> Oops, you explained it in part 0.
> 
> > Step 5 - Final Switch to Destination
> > ====================================
> > VM1 is now able to switch to the destination for both read and
> > write
> > operations.
> > 
> > [src/hd0base] <- [src/hd0snap1] <= VM1(read-write)
> > 
> > (qemu) snapshot_blkdev -n ide0-hd0 /tmp/dst/hd0snap1.qcow2
> 
> It seems to me that reusing the snapshot_blkdev command is a bit of a
> misnomer.  It also forces you to use format autodetection for the
> destination in the blkmirror phase.
> 
> I see two possibilities:
> 
> 1) you can make a new command block_reopen; this fixes only the
> naming
> problem.
> 
> 2) you can hide the new format behind a new command to be used like
> this:
> 
>    block_migrate --mirror ide-hd0 /tmp/dst/hd0snap1.qcow2
>    ...
>    block_migrate --switch ide-hd0
> 
> This leave the possibility to specify the format in the future:
> 
>    block_migrate --mirror ide-hd0 /tmp/dst/hd0snap1.qcow2 qcow2
> 
> And we could have another sub-command
> 
>    block_mirror --stream ide-hd0 /tmp/dst/hd0.raw
> 
> to migrate block devices without shared storage and without an
> intermediate snapshot.

I agree on the fact that using snapshot_blkdev is a misnomer but at
the same time I like very much how blkmirror is modular and it can
be used as a regular image filename.
I'm worried that making it explicit with a "--mirror" option would
take away its flexibility.
What about adding block_reopen and extending the blkmirror string:

 (qemu) block_reopen ide-hd0 \
 blkmirror:qcow2:/tmp/src/hd0snap1.qcow2:/tmp/dst/hd0snap1.qcow2

so that the format is passed as first argument (and if it's empty we
would use the auto-detection).

-- 
Federico