Re: [Qemu-devel] PATCH: v3 Allow control over drive file open mode

[Jamie Lokier]

Anthony Liguori wrote:
> Down the road, you do a savevm.  savevm has to create a checkpoint in 
> all disk images.  The checkpoint will be empty in boo.img but it still 
> needs to create one.  An alternative scenario would be a read-only root fs:
> 
> qemu-system-x86_64 -drive file=rootfs.img,read-only=on
> 
> Where you wanted to be able to do a savevm.

I agree this could be useful (despite my wariness of snapshots :)

In which case, it would be all-the-more useful if it worked correctly
when multiple QEMU instances are using the image file.

"Down the road" - you might not know in advance that you're wishing to
save a snapshot, and by then you might have other running QEMU
instances using that image which you don't want to kill, and of course
if you can't snapshot one, you can't snapshot any of them to shut them
down temporarily.

It should be enough to take an advisory lock during the savevm writes,
shouldn't it?

-- Jamie