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

[Lukas Czerner]

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.

Thanks!
-Lukas
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