[dm-devel] [PATCH V3 0/5] dm-rq: improve sequential I/O performance

[Mike Snitzer]

On Fri, Jan 12 2018 at  1:54pm -0500,
Bart Van Assche <Bart.VanAssche at wdc.com> wrote:

> On Fri, 2018-01-12 at 13:06 -0500, Mike Snitzer wrote:
> > OK, you have the stage: please give me a pointer to your best
> > explaination of the several.
> 
> Since the previous discussion about this topic occurred more than a month
> ago it could take more time to look up an explanation than to explain it
> again. Anyway, here we go. As you know a block layer request queue needs to
> be rerun if one or more requests are waiting and a previous condition that
> prevented the request to be executed has been cleared. For the dm-mpath
> driver, examples of such conditions are no tags available, a path that is
> busy (see also pgpath_busy()), path initialization that is in progress
> (pg_init_in_progress) or a request completes with status, e.g. if the
> SCSI core calls __blk_mq_end_request(req, error) with error != 0. For some
> of these conditions, e.g. path initialization completes, a callback
> function in the dm-mpath driver is called and it is possible to explicitly
> rerun the queue. I agree that for such scenario's a delayed queue run should
> not be triggered. For other scenario's, e.g. if a SCSI initiator submits a
> SCSI request over a fabric and the SCSI target replies with "BUSY" then the
> SCSI core will end the I/O request with status BLK_STS_RESOURCE after the
> maximum number of retries has been reached (see also scsi_io_completion()).
> In that last case, if a SCSI target sends a "BUSY" reply over the wire back
> to the initiator, there is no other approach for the SCSI initiator to
> figure out whether it can queue another request than to resubmit the
> request. The worst possible strategy is to resubmit a request immediately
> because that will cause a significant fraction of the fabric bandwidth to
> be used just for replying "BUSY" to requests that can't be processed
> immediately.
> 
> The intention of commit 6077c2d706097c0 was to address the last mentioned
> case. It may be possible to move the delayed queue rerun from the
> dm_queue_rq() into dm_requeue_original_request(). But I think it would be
> wrong to rerun the queue immediately in case a SCSI target system returns
> "BUSY".

OK, thank you very much for this.  Really helps.

For starters multipath_clone_and_map() could do a fair amount more with
the insight that a SCSI "BUSY" was transmitted back.  If both blk-mq
being out of tags and SCSI "BUSY" simply return BLK_STS_RESOURCE then
dm-mpath doesn't have the ability to behave more intelligently.

Anyway, armed with this info I'll have a think about what we might do to
tackle this problem head on.

Thanks,
Mike