Re: [Qemu-devel] [PATCH 05/18] ide: Turn debug messages into assertions

[Markus Armbruster <armbru@redhat.com>]

Kevin Wolf <kwolf@redhat.com> writes:

> Am 06.06.2011 13:57, schrieb Markus Armbruster:
>>>>> Not sure what's the best way of fixing this. Maybe just ignoring
>>>>> -snapshot for read-only block devices?
>>>>
>>>> Why not, the combination is pointless.
>>>
>>> It could start making a difference in some obscure combinations. Imagine
>>> a read-only image with a backing file, -snapshot and the 'commit'
>>> monitor command.
>>>
>>> Sounds pretty insane, but I wouldn't bet that people aren't using it...
>> 
>> People try all kinds of insane things.  The question is whether we can
>> change it anyway.
>
> We have a backing file chain like base <- cow [<- tmp], and the drive is
> read-only.
>
> Currently, 'commit' means that tmp is committed to cow (i.e. nothing
> happens because it's read-only). After changing it, we would commit the
> content of cow to base and possibly corrupt other images that are based
> on base.
>
> We can hope that nobody would be hit by it in practice, but it's not a
> change I'd feel very comfortable about.

So the one effect -snapshot has on a read-only drive is to neuter the
commit command.  Hmm.

Naive question: shouldn't commit require the drive to be read/write?  It
writes both the backing image and the COW...

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