Re: [Qemu-devel] [PATCH 1/7] block: only force IO completion in .bdrv_truncate if we are shrinking

[Kevin Wolf <kwolf@redhat.com>]

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