[dm-devel] multipath and READ_CAPACITY

Hannes Reinecke hare at suse.de
Thu Jul 24 08:46:31 UTC 2014

Hi Christophe,

I've had a customer issue where someone complained that multipath 
would reject paths if they are in ALUA 'standby'.
As it turns out, SPC-3 does _not_ require the READ_CAPACITY command 
to be implemented if a target port is in standby.
Consecutively, linux would register this path with a capacity of 
'0', and multipath would reject the path in ev_add_path().

Which makes we wonder, do we actually _need_ the check for a matched 
With VPD page 0x83 pretty much standard as of now (and I'm reasonbly 
sure we'll reject any device _not_ providing VPD page 0x83 anyway) 
we basically have to rely on that information.
IE if the WWID is identical we have assume the devices are 
identical. So it would make sense to just remove the check altogether.



Dr. Hannes Reinecke		      zSeries & Storage
hare at suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)

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