linux-kernel - Re: [PATCH 1/2] blkdev: Submit discard bio in batches in blkdev_issue

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Date:	Thu, 28 Apr 2011 09:29:29 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Lukas Czerner <lczerner@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Dmitry Monakhov <dmonakhov@...nvz.org>, jaxboe@...ionio.com
Subject: Re: [PATCH 1/2] blkdev: Submit discard bio in batches in blkdev_issue_discard()

Lukas Czerner <lczerner@...hat.com> writes:

> On Thu, 28 Apr 2011, Lukas Czerner wrote:
>
>> On Wed, 27 Apr 2011, Jeff Moyer wrote:
>> 
>> > Lukas Czerner <lczerner@...hat.com> writes:
>> > 
>> > > 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.
>> > 
>> > I don't see any major problems here.  I would like you to test
>> > explicitly for queue_flag_discard before submitting any bios, though.
>
> Btw, this is already in there:
>
> 	if (!blk_queue_discard(q))
> 		return -EOPNOTSUPP;
>
> at the top of the function.

Boy, how did I miss that?  ;-)

Cheers,
Jeff
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