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

[Markus Armbruster]

Fam Zheng <address@hidden> writes:

> 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?

Maybe, I don't know.

>        If not, can we try to detect such case  on opening

Gee, what a nice swamp you found there!

For local files, you can compare (dev-major, dev-minor, inode).

Beyond that, you tend not to get comparisons, but best guesses.

>                                                           and try to
> reuse the bs?

I doubt reusing the BDS is correct in the general case.

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