Re: scsi_dh_alua: add missing transitioning state support

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Aug 30 2010 at  5:36am -0400,
Hannes Reinecke <hare@suse.de> wrote:

> Nicholas A. Bellinger wrote:
> > On Tue, 2010-08-17 at 15:05 -0400, Mike Snitzer wrote:
> >> Handle transitioning in the prep_fn.
> >> Handle transitioning in alua_rtpg's implicit alua code too.
> >>
> >> These gaps were identified during controller failover testing of an
> >> ALUA array.
> >>
> >> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> >> ---
> >>  drivers/scsi/device_handler/scsi_dh_alua.c |   10 +++++++---
> >>  1 files changed, 7 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/scsi/device_handler/scsi_dh_alua.c b/drivers/scsi/device_handler/scsi_dh_alua.c
> >> index 1a970a7..c1eedc5 100644
> >> --- a/drivers/scsi/device_handler/scsi_dh_alua.c
> >> +++ b/drivers/scsi/device_handler/scsi_dh_alua.c
> >> @@ -616,6 +616,9 @@ static int alua_rtpg(struct scsi_device *sdev, struct alua_dh_data *h)
> >>  		    h->state == TPGS_STATE_STANDBY)
> >>  			/* Useable path if active */
> >>  			err = SCSI_DH_OK;
> >> +		else if (h->state == TPGS_STATE_TRANSITIONING)
> >> +			/* State transition, retry */
> >> +			goto retry;
> >>  		else
> >>  			/* Path unuseable for unavailable/offline */
> >>  			err = SCSI_DH_DEV_OFFLINED;
> >> @@ -698,13 +701,14 @@ static int alua_prep_fn(struct scsi_device *sdev, struct request *req)
> >>  	struct alua_dh_data *h = get_alua_data(sdev);
> >>  	int ret = BLKPREP_OK;
> >>  
> >> -	if (h->state != TPGS_STATE_OPTIMIZED &&
> >> -	    h->state != TPGS_STATE_NONOPTIMIZED) {
> >> +	if (h->state == TPGS_STATE_TRANSITIONING)
> >> +		ret = BLKPREP_DEFER;
> >> +	else if (h->state != TPGS_STATE_OPTIMIZED &&
> >> +		 h->state != TPGS_STATE_NONOPTIMIZED) {
> >>  		ret = BLKPREP_KILL;
> >>  		req->cmd_flags |= REQ_QUIET;
> >>  	}
> >>  	return ret;
> >> -
> >>  }
> >>  
> > 
> > Makes sense to me..
> > 
> > Acked-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
> > 
> Not so fast. There are two problems with this approach:
> 
> The path is retried indefinitely. Arrays are _supposed_ to be in 'transitioning'
> only temporary; however, if the array is stuck due to a fw error we're stuck in 'defer',
> too.

And what is the problem with that?  The IO will eventually time out.

> Secondly this path fails with 'directio' multipath checker. Remember that 'directio'
> is using 'fs' requests, not block-pc ones. Hence for all I/O the prep_fn() callback
> is evaluated, which will return 'DEFER' here once the path is in transitioning.
> And the state is never updated as RTPG is never called.

Testing ALUA with directio path checker did not result in such immutable
state in the few instances that TPGS_STATE_TRANSITIONING was seen in
alua_prep_fn.

> I'm currently preparing a patch which addressed these situations, too.

OK, please share.

Thanks,
Mike