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SubjectRe: [PATCH 1/3] blkdev: Submit discard bio in batches in blkdev_issue_discard()
On 2011-05-03 03:30, Lukas Czerner wrote:
> On Mon, 2 May 2011, Jens Axboe wrote:
>
>> On 2011-05-02 08:17, Lukas Czerner wrote:
>>> Currently we are waiting for every submitted REQ_DISCARD bio separately,
>>> but it can have unwanted consequences of repeatedly flushing the queue,
>>> so we rather submit bios in batches and wait for the entire batch, hence
>>> narrowing the window of other ios going in.
>>>
>>> Use bio_batch_end_io() and struct bio_batch for that purpose, the same
>>> is used by blkdev_issue_zeroout(). Also change bio_batch_end_io() so we
>>> always set !BIO_UPTODATE in the case of error and remove the check for
>>> bb, since we are the only user of this function and we always set this.
>>>
>>> Remove bio_get()/bio_put() from the blkdev_issue_discard() since
>>> bio_alloc() and bio_batch_end_io() is doing the same thing, hence it is
>>> not needed anymore.
>>>
>>> I have done simple dd testing with surprising results. The script I have
>>> used is:
>>>
>>> for i in $(seq 10); do
>>> echo $i
>>> dd if=/dev/sdb1 of=/dev/sdc1 bs=4k &
>>> sleep 5
>>> done
>>> /usr/bin/time -f %e ./blkdiscard /dev/sdc1
>>>
>>> Running time of BLKDISCARD on the whole device:
>>> with patch without patch
>>> 0.95 15.58
>>>
>>> So we can see that in this artificial test the kernel with the patch
>>> applied is approx 16x faster in discarding the device.
>>
>> Good results. It'd be more efficient to add the vectored discard support
>> and use it for this too though, and it would get rid of the need to wait
>> on the batches since there'd be just one discard bio for the ranges.
>>
>
> I know that you and Christoph were working on adding support for
> vectored discard, but I do not know what is the status of it ?

No new status. I sent a test patch to Christoph and that's about it on
that end for now. But it could be expedited, if need be.

--
Jens Axboe



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