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Re: [dm-devel] [multipath] SCSI device capacity mess



> Switching LUNs from one path to another is not a fast process and if
> this happens during periods of heaving IO periods, significant thrashing
> can result.
> 
> I think if we want to play at the Enterprise level where there are many
> servers connected up to large arrays I don't think it is a good idea to
> knowingly cause disruptions to others on the SAN.  Causing LUNs to
> switch from one path to another is such a behavior.
> 
I certainly don't militate for the NOSTARTONADD flag removal for all
devices. I just consider it annoying for the HSG80 / HSV* family
controlers :

- Either they are in failover mode and there is no bouncing possible, so
start_stop is harmless

- Either they are in multibus mode, then not sending start_stop before
READ CAPA lead to cmd failure and wrong size stored. Hence ghosts path
are ununsable even when they come up. [Here, I shamely hide the
device-mapper hooks on path activation could work]

- Either way, these controler families are *not* high-end devices,
whatever the criteria (capacity, throughput, cache, alarming, ...). The
whole ghost-path notion being the best evidence.

regards,
-- 
christophe varoqui <christophe varoqui free fr>


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