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

[Markus Armbruster]

Fam Zheng <address@hidden> writes:

> On Tue, 06/18 16:18, Markus Armbruster wrote:
>> 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?
>
> It's never sufficient by comparing filename to tell if they are the
> same, things can be tricky here, but in many cases it can be helpful,
> both local and remote.

Let me rephrase my question.

We all understand that different bs->filename can alias the same
resource (which is not necessarily a file).  This makes a "same
resource" test based on bs->filename incomplete.

Does identical bs->filename *always* imply same resource?

If yes, the test is correct but incomplete.  That can be useful.

If no, the test is incorrect and incomplete, thus useless.

>> If it's a local file, then comparing names to check for aliasing is
>> stupid.  Compare device & inode instead.
>
> Device and inode is not something to block layer's knowledge, I think.

They are one stat(2) or fstat(2) away.