Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert

[Paolo Bonzini <pbonzini@redhat.com>]

Il 17/07/2013 12:27, Kevin Wolf ha scritto:
> Am 17.07.2013 um 12:21 hat Peter Lieven geschrieben:
>>
>> Am 17.07.2013 um 11:58 schrieb Paolo Bonzini <pbonzini@redhat.com>:
>>
>>> Il 17/07/2013 10:46, Kevin Wolf ha scritto:
>>>> Am 15.07.2013 um 12:49 hat Peter Lieven geschrieben:
>>>>> if a destination has has_zero_init = 0, but it supports
>>>>> discard zeroes use discard to convert the target
>>>>> into an all zero device.
>>>>>
>>>>> Signed-off-by: Peter Lieven <pl@kamp.de>
>>>>
>>>> Wouldn't it be better to use bdrv_write_zeroes() and extend the
>>>> implementation of that to use discard internally in those block drivers
>>>> where it makes sense?
>>>>
>>>> Because here you're not really discarding (i.e. don't care about the
>>>> sectors any more), but you want them to be zeroed.
>>>
>>> I thought the same yesterday when reviewing the series, but I'm not
>>> convinced.
>>>
>>> Discarding is not always the right way to write zeroes, because it can
>>> disrupt performance.  It may be fine when you are already going to write
>>> a sparse image (as is the case for qemu-img convert), but not in
>>> general.  So if you just used write_zeroes, it would have to fall under
>>> yet another -drive option (or an extension to "-drive discard").  I
>>> think what Peter did is a good compromise in the end.
>>>
>>> BTW, Peter and Ronnie: we were assuming that UNMAP with LBPRZ=1 always
>>> zeroes blocks, but is that true for unaligned operations?
>>
>> Good question, I will pass it to ronnie. My guess is that the command will fail with
>> a check condition if it failed to unmap the data. From what Ronnie sent earlier
>> it should be guaranteed that the blocks are at least zero after the unmap command.
>>
>> As for the qemu-img patch this shouldn't matter. It uses always blocks of bdi->max_unmap
>> which should be a multiple of the alignment. It also checks if sectors are deallocated
>> after the unmap afterwards. If the unmap fails it falls back to has_zero_init =1.
> 
> Well, you use bdrv_discard(), and ignoring discards is valid. Just
> another reason to use bdrv_write_zeroes() instead.

He's only using it if discard_zeroes is true in the new BlockDriverInfo.
 We can define the semantics of that bit, and I think defining it as
"ignored discards will still write zeroes" is a good thing (same as what
SCSI targets do if you use WRITE SAME to do the discard operation).

Paolo