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Re: [Qemu-block] [Qemu-devel] [PULL 08/17] block: Assert no write reques

From: Kevin Wolf
Subject: Re: [Qemu-block] [Qemu-devel] [PULL 08/17] block: Assert no write requests under BDRV_O_INCOMING
Date: Fri, 19 Feb 2016 15:17:31 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

Am 19.02.2016 um 15:11 hat Paolo Bonzini geschrieben:
> On 19/02/2016 14:47, Kevin Wolf wrote:
> > > The problem seems coming from "snapshot=on".
> > Aha. The combination of migration with snapshot=on is obvious nonsense
> > (because on the destination the disk state would be reverted while the
> > guest OS thinks the disk is unchanged),
> Not really, on the destination the disk state will not be _further
> updated_, but no reversion happens.

Right, I started writing a patch when I noticed that that wasn't quite
right and -snapshot on the destination is somewhat unusual, but not in
all cases wrong.

> Migration with "snapshot=on" makes sense if you are resuming a VM from a
> saved checkpoint ("-incoming exec").  You can use it to debug a
> migration load, for example, because you want the resumed guest to
> always start with the same disk contents.  It's somewhat weird, but it
> definitely has its uses.  A blocker would be a pain in the ass...

My other thinko was that a blocker would help. In reality, blockers are
on the source host. They would make a little more sense than forbidding
-snapshot with -incoming, but we don't know how you migrate storage. If
you don't use shared storage but mirroring, then it could be okay again
(but probably even weirder than the other case).


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