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Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on c
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert |
Date: |
Wed, 17 Jul 2013 12:47:58 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7 |
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 <address@hidden>:
>>
>>> 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 <address@hidden>
>>>>
>>>> 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
- [Qemu-devel] [PATCH 0/4] qemu-img: conditionally discard target on convert, Peter Lieven, 2013/07/15
- [Qemu-devel] [PATCH 1/4] block: add discard_zeroes and max_unmap to BlockDriverInfo, Peter Lieven, 2013/07/15
- [Qemu-devel] [PATCH 3/4] block/raw: add .bdrv_get_info, Peter Lieven, 2013/07/15
- [Qemu-devel] [PATCH 2/4] iscsi: add .bdrv_get_info, Peter Lieven, 2013/07/15
- [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Peter Lieven, 2013/07/15
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Kevin Wolf, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Paolo Bonzini, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Peter Lieven, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Kevin Wolf, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Peter Lieven, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert,
Paolo Bonzini <=
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Peter Lieven, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Paolo Bonzini, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Peter Lieven, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Paolo Bonzini, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Kevin Wolf, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, ronnie sahlberg, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Paolo Bonzini, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, ronnie sahlberg, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Peter Lieven, 2013/07/17
- Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert, Paolo Bonzini, 2013/07/17