Re: [PATCH 1/3] blkdev: Submit discard bio in batches in blkdev_issue_discard()

[Jens Axboe <jaxboe@fusionio.com>]

On 2011-05-05 09:20, 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.
>>
> 
> Since we do not have vectored discard yet, can this change go in as it
> is ? Also, Christoph thinks that it would not help anyway since the we do
> batch the discard requests only when it exceeds the max range.

Sure, it need not be a show stopper. I will apply 1-2 for now to
for-2.6.40/core.

-- 
Jens Axboe