Re: [Qemu-devel] [PATCH] block: add 'backing' option to drive_add

[Markus Armbruster]

Fam Zheng <address@hidden> writes:

> On Tue, 06/18 08:32, Kevin Wolf wrote:
>> Am 18.06.2013 um 05:58 hat Fam Zheng geschrieben:
>> > On Mon, 06/17 17:12, Kevin Wolf wrote:
>> > > Am 17.06.2013 um 16:46 hat Paolo Bonzini geschrieben:
>> > > > Il 17/06/2013 16:26, Kevin Wolf ha scritto:
>> > > > > Am 17.06.2013 um 16:01 hat Paolo Bonzini geschrieben:
>> > > > >> Il 17/06/2013 15:52, Kevin Wolf ha scritto:
>> > > > >>> It's not a new thought that we need to change the block
>> > > > >>> layer so that a
>> > > > >>> BlockDriverState can't be "empty", but that one
>> > > > >>> BlockDriverState always
>> > > > >>> refers to one image. If you change media, you attach a different
>> > > > >>> BlockDriverState to the device. Once you have this, you can start
>> > > > >>> refcounting BlockDriverStates, so that the backing file
>> > > > >>> remains usable
>> > > > >>> while the guest device already uses a different image.
>> > > > >>>
>> > > > >>> Not that it's it easy to get there...
>> > > > >>
>> > > > >> I'm not sure that is safe to do.
>> > > > >>
>> > > > >> Consider the case where the guest switches from A to B during 
>> > > > >> backup,
>> > > > >> and then from B to A.  You get two BDS for the same file,
>> > > > >> which pretty
>> > > > >> much means havoc.
>> > > > > 
>> > > > > Well, yes, it means that the management tool needs to know what it's
>> > > > > doing. It shouldn't create a second BDS for A, but reattach the still
>> > > > > existing one.
>> > > > 
>> > > > How?  That would require the management tool to know the full chain of
>> > > > BDSes that were opened in the past.
>> > > 
>> > > They better know on which files they are operating. It's not like the
>> > > management could be unaware of running backup jobs or things like that.
>> > > 
>> > 
>> > Is there any case that QEMU needs to have two BDS pointing to the same
>> > file?
>> 
>> No, I think there's no case where this would make sense.
>> 
>> > If not, can we try to detect such case  on opening and try to
>> > reuse the bs?
>> 
>> We can't do it reliably, think about symlinks or even hard links, or
>> things like /dev/fdset/..., let alone remote protocols that refer to the
>> same image file etc.
>> 
>> We can check the obvious cases and error out for them, but that's about
>> what we can do. I don't think we should try to fix things automagically
>> when we can't do it right.
>
> It's impossible to know a remote protocol points to the same image with
> local file path, that's not in QEMU's scope, but we have a good chance
> to detect (strcmp with existing bs->filename) and error out Paolo's
> A-B-A problem, don't we?

Is comparing bs->filename always a good idea, or only if it's a local
image file?

If it's a local file, then comparing names to check for aliasing is
stupid.  Compare device & inode instead.