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

[Paolo Bonzini]

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.
> 
> 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.

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.

Paolo