Re: [PATCH] scsi_dh_alua: add missing transitioning state support

[Hannes Reinecke <hare@suse.de>]

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.

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.

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

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: Markus Rex, HRB 16746 (AG Nürnberg)
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