Am 06.03.2013 um 21:39 hat Paolo Bonzini geschrieben: > Il 06/03/2013 20:03, Peter Lieven ha scritto: > > Am 06.03.2013 19:48, schrieb Jeff Cody: > >> On Wed, Mar 06, 2013 at 07:31:51PM +0100, Paolo Bonzini wrote: > >>> Il 06/03/2013 19:14, Jeff Cody ha scritto: > >>>> QCOW breaks with it using a normal raw posix file as a device. As a > >>>> test: qemu-img create -f qcow test.qcow 5G. Now run qemu with that > >>>> drive mounted, and try to partition and format it. QEMU now asserts. > >>>> > >>>> The nicety of being able to using truncate during a write call, > >>>> especially for VHDX (which can have relatively large block/cluster > >>>> sizes), so to grow the file sparsely in a dynamically allocated file. > >>> > >>> Perhaps we need two APIs, "truncate" and "revalidate". > >>> > >>> Truncate should be a no-op if (!bs->growable). > >>> > >>> Revalidate could be called by the block_resize monitor command with no > >>> size specified. > >>> > >>> Paolo > >> > >> I think that is a good solution. Is it better to have "truncate" and > >> "revalidate", or "truncate" and "grow", with grow being a subset of > >> truncate, with fewer restrictions? There may still be operations > >> where it is OK to grow a file, but not OK to shrink it.
What semantics would the both operations have? Is truncate the same as it used to be? I don't really understand what "revalidate" would do, it sounds like a read-only operation from its name? > > Or as a first step: > > > > a) Call brdv_drain_all() only if the device is shrinked (independently of > > !bs->growable) > > b) Call brdv_drain_all() inside iscsi_truncate() because it is a special > > requirement there > > c) Fix the value of bs->growable for all drivers > > Let's start from (c). bdrv_file_open sets bs->growable = 1. I think it > should be removed and only the file protocol should set it. This is probably right. > Then we can add bdrv_revalidate and, for block_resize, call > bdrv_revalidate+bdrv_truncate. For bs->growable = 0 && > !bs->drv->bdrv_truncate, bdrv_truncate can just check that the actual > size is the same or bigger as the one requested, and fail otherwise. This one not so much. bs->growable does not mean that you can use bdrv_truncate. It rather means that you may write beyond the end of the file even without truncating it first. Mabye bs->auto_grow would be a better for it. So bs->growable == true implies that bdrv_truncate() should be allowed as well, because obviously changing the BDS size is possbile, but it's not true the other way round. Kevin