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Re: [Qemu-block] [PATCH 1/1] blk: do not select PFLASH device for intern

From: Kevin Wolf
Subject: Re: [Qemu-block] [PATCH 1/1] blk: do not select PFLASH device for internal snapshot
Date: Tue, 12 Jan 2016 16:20:51 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

Am 12.01.2016 um 15:59 hat Paolo Bonzini geschrieben:
> On 12/01/2016 15:16, Kevin Wolf wrote:
> >> Thus we should avoid selection of "pflash" drives for VM state saving.
> >>
> >> For now "pflash" is read-write raw image as it configured by libvirt.
> >> Thus there are no such images in the field and we could safely disable
> >> ability to save state to those images inside QEMU.
> > 
> > This is obviously broken. If you write to the pflash, then it needs to
> > be snapshotted in order to keep a consistent state.
> > 
> > If you want to avoid snapshotting the image, make it read-only and it
> > will be skipped even today.
> Sort of.  The point of having flash is to _not_ make it read-only, so 
> that is not a solution.
> Flash is already being snapshotted as part of saving RAM state.  In 
> fact, for this reason the device (at least the one used with OVMF; I 
> haven't checked other pflash devices) can simply save it back to disk 
> on the migration destination, without the need to use "migrate -b" or 
> shared storage.
> [...]
> I don't like very much using IF_PFLASH this way, which is why I hadn't
> replied to the patch so far---I hadn't made up my mind about *what* to
> suggest instead, or whether to just accept it.  However, it does work.
> Perhaps a separate "I know what I am doing" skip-snapshot option?  Or
> a device callback saying "not snapshotting this is fine"?

Boy, is this ugly...

What do you do with disk-only snapshots? The recovery only works as long
as you have VM state.


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