linux-kernel - Re: [PATCH 1/3] blkdev: Submit discard bio in batches in blkdev_issue

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Date:	Fri, 6 May 2011 19:23:13 -0600
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Lukas Czerner <lczerner@...hat.com>
CC:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dmitry Monakhov <dmonakhov@...nvz.org>,
	Jeff Moyer <jmoyer@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [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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