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

[Peter Lieven <pl@dlhnet.de>]

On 06.03.2013 21:39, Paolo Bonzini wrote:
> 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
>

Regarding brd_drain_all(). Is the fix right to call it only on device shrink?
In this case it has to be added to iscsi_truncate as well.

Peter