[dm-devel] [PATCH] dm-mpath: always return reservation conflict

[Hannes Reinecke]

On 07/15/2015 01:56 PM, Christoph Hellwig wrote:
> An array can't issue a reservation, the initiator needs to register
> it.  Right now the only way to do it is through SG_IO passthrough,
> which is a best luck effort it I/O isn't also using SG_IO and can't
> be properly supported because of that.
> 
> However I will submit an in-kernel reservation API soon which will
> allow us to have that sort of control.  My current prototyp only allows
> for all-path reservations as I couldn't come up with a use case for
> per-path reservations, but if such a need should arise we can add it
> and take that into account in the multipathing code.
> 
Which was my reasoning as well.
I would consider a per-path reservation in a multipath setup an
error, as the current multipath code is not able to handle this.
With the current code we will fail a path due to the reservation
conflict error, but whatever happens next depends on the type of
reservation and the used prioritizer/path checker.
It can be everything from 'just working' to recurrent path drops to
and I/O stall (as SET TARGET PORT GROUPS might return an reservation
conflict, too, so we wouldn't be able to switch to a working path...)

And implementing a per-path reservation in multipath is far from
trivial, so I'd rather not attempt this.
_Especially_ not as you're working on a in-kernel reservation code.

Cheers,

Hannes
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