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Re: [Qemu-block] [PATCH] mirror: Improve zero-write and discard with fra
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-block] [PATCH] mirror: Improve zero-write and discard with fragmented image |
Date: |
Tue, 10 Nov 2015 10:01:54 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 |
On 10/11/2015 07:14, Fam Zheng wrote:
> On Mon, 11/09 17:29, Kevin Wolf wrote:
>> Am 09.11.2015 um 17:18 hat Paolo Bonzini geschrieben:
>>>
>>>
>>> On 09/11/2015 17:04, Kevin Wolf wrote:
>>>> Am 06.11.2015 um 11:22 hat Fam Zheng geschrieben:
>>>>> The "pnum < nb_sectors" condition in deciding whether to actually copy
>>>>> data is unnecessarily strict, and the qiov initialization is
>>>>> unnecessarily too, for both bdrv_aio_write_zeroes and bdrv_aio_discard
>>>>> branches.
>>>>>
>>>>> Reorganize mirror_iteration flow so that we:
>>>>>
>>>>> 1) Find the contiguous zero/discarded sectors with
>>>>> bdrv_get_block_status_above() before deciding what to do. We query
>>>>> s->buf_size sized blocks at a time.
>>>>>
>>>>> 2) If the sectors in question are zeroed/discarded and aligned to
>>>>> target cluster, issue zero write or discard accordingly. It's done
>>>>> in mirror_do_zero_or_discard, where we don't add buffer to qiov.
>>>>>
>>>>> 3) Otherwise, do the same loop as before in mirror_do_read.
>>>>>
>>>>> Signed-off-by: Fam Zheng <address@hidden>
>>>>
>>>> I'm not sure where in the patch to comment on this, so I'll just do it
>>>> here right in the beginning.
>>>>
>>>> I'm concerned that we need to be more careful about races in this patch,
>>>> in particular regarding the bitmaps. I think the conditions for the two
>>>> bitmaps are:
>>>>
>>>> * Dirty bitmap: We must clear the bit after finding the next piece of
>>>> data to be mirrored, but before we yield after getting information
>>>> that we use for the decision which kind of operation we need.
>>>>
>>>> In other words, we need to clear the dirty bitmap bit before calling
>>>> bdrv_get_block_status_above(), because that's both the function that
>>>> retrieves information about the next chunk and also a function that
>>>> can yield.
>>>>
>>>> If after this point the data is written to, we need to mirror it
>>>> again.
>>>
>>> With Fam's patch, that's not trivial for two reasons:
>>>
>>> 1) bdrv_get_block_status_above() can return a smaller amount than what
>>> is asked.
>>>
>>> 2) the "read and write" case can handle s->granularity sectors per
>>> iteration (many of them can be coalesced, but still that's how the
>>> iteration works).
>>>
>>> The simplest solution is to perform the query with s->granularity size
>>> rather than s->buf_size.
>>
>> Then we end up with many small operations, that's not what we want.
>>
>> Why can't we mark up to s->buf_size dirty clusters as clean first, then
>> query the status, and mark all of those that we can't handle dirty
>> again?
>
> Then we may end up marking more clusters as dirty than it should be.
You're both right.
> Because all bdrv_set_dirty() and bdrv_set_dirty_bitmap() callers are
> coroutine,
> we can introduce a CoMutex to let bitmap reader block bdrv_set_dirty and
> bdrv_set_dirty_bitmap.
I think this is not necessary.
I think the following is safe:
1) before calling bdrv_get_block_status_above(), find out how many
consecutive bits in the dirty bitmap are 1
2) zero all those bits in the dirty bitmap
3) call bdrv_get_block_status_above() with a size equivalent to the
number of dirty bits
4) if bdrv_get_block_status_above() only returns a partial result, loop
step (3) until all the dirty bits are processed
For full mirroring, this strategy will probably make the first
incremental iteration more expensive.
Paolo