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

    Jens Axboe

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