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

[Jeff Cody]

On Wed, Mar 06, 2013 at 07:06:24PM +0100, Paolo Bonzini wrote:
> Il 06/03/2013 18:50, Peter Lieven ha scritto:
> >> > Commit 9a665b2b made bdrv_truncate() call bdrv_drain_all(), but this 
> >> > breaks
> >> > QCOW images, as well other future image formats (such as VHDX) that may 
> >> > call
> >> > bdrv_truncate(bs->file) from within a read/write operation.  For 
> >> > example, QCOW
> >> > will cause an assert, due to tracked_requests not being empty (since the
> >> > read/write that called bdrv_truncate() is still in progress).
> 
> I'm not sure such bdrv_truncate calls are necessary.  QCOW2 doesn't have
> them (almost; there is one in qcow2_write_compressed, I'm not even sure
> that one is necessary though), and I think QCOW's breaks using it with a
> block device as a backing file.
> 
> Paolo

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.