On 08/16/2012 07:35 AM, Kevin Wolf wrote: > Am 16.08.2012 14:56, schrieb Eric Blake: >> On 08/16/2012 04:00 AM, Peter Maydell wrote: >>> On 16 August 2012 10:00, Kevin Wolf <kw...@redhat.com> wrote: >>>> People have repeatedly expected that you can do things like snapshotting >>>> an image with qemu-img while a qemu instance is running. Maybe we need >>>> to consider locking the files while they are in use, >> >> Sounds like a nice feature bit to add to qcow2v3, where both qemu-img >> and qemu check if the locking feature is enabled for an image, as well >> as maintain a header bit that is set when the image is open read-write >> and refuse to use the image if the lock bit is set. > > I thought the same. However, then you need some way to override this > mechanism when recovering from a crash etc., so it's not a trivial > addition and it would be user-visible.
Good point. With fcntl() locking, the lock goes away on crash; but with file modification locking, the lock can get stuck so you have to provide overrides; and once you provide overrides, the lock is not quite as powerful. So maybe it's best to just leave locking up to management apps (after all, libvirt already has a lock protocol support, currently built on sanlock but also with a patch proposed for using fcntl()). >> Is this strong enough? Remember, with qcow2v3 and qed, the mere act of >> opening an image will perform refcount checks that modify the image, >> unless you explicitly request otherwise, which means even a query of the >> file metadata may result in modifying the image as part of the default >> open. > > Not for read-only opens. I think qemu-img gets this right meanwhile, so > that images are opened read-only when they are only queried. Then it might also be worth documenting which actions are read-only queries vs. potential modifications, in the docs for each action. (For example, 'info' is read-only, 'convert' is read-only when creating a copy although consistency is essential for it to be useful, 'check' is readonly while 'check -r' is read-write,...) -- Eric Blake ebl...@redhat.com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
signature.asc
Description: OpenPGP digital signature