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Re: [dm-devel] device-errors and multipath device access issue

* Tore Anderson

Maybe it has a way of being configured in a fake active/active
configuration where I/O can be processed on both controllers at the
same time.  I believe newer EMC CLARiiON and HP EVA can be set up in
ALUA mode which does the trick. HDS AMS does something similar which
work mostly the same way.

* devzero web de

yes - but i don`t think the admin wants to touch such essential
configuration to get rid of error messages in the kernel-log....

Today I played around with a CX500 in EMC's lab (I'm considering buying another CLARiiON), and I got to test the ALUA mode quite a bit. It works very well, and I'd recommend you to pester your admin to get it.

Basically what it does is that if I/O is received on the passive controller, it's just redirected to the active controller on an internal interconnect (and the replies are routed the same way back out again). So it looks and feels just like a true active/active setup, you'll not get I/O errors on any of the paths. Supposedly there is a slight increase in latency on I/O that pass through the passive controller, but that was not noticeable in my simple benchmarks using multibus topology.

The system was dedicated to my tests, though, so I'm not promising it's okay to be using multibus in production on a busy system. A better way to set it up is to be using mpath_prio_emc and group_by_prio like before, but with no hardware handler. If dm-multipath decides to change priority groups the I/O will simply pass through the passive controller for some minutes until the CX realises that most of the I/O is going in the wrong way, and trespasses the volume automatically. Small bursts of stray I/O to the passive controller (like those your proprietary application causes) will be processed fine (with the very slight latency increase), but it won't affect the production I/O going through dm-multipath straight to the active controller. It's really nice for use with dm-multipath - and it'll solve all of your problems.

There's an ALUA hardware handler in the making too - I assume it will make it possible for dm-multipath to explicitly trespass the volume on PG change (instead of relying on the CX to automatically move the LUN). In my opinion, though, that's not needed for a production quality setup with ALUA on the CLARiiON (maybe it's mostly inteded for other ALUA-capable arrays that might not have the auto-trespass feature).

ALUA mode is configured on a per-host basis (set it to failover mode 4, CLARiiON Open), so your admin shouldn't worry about changing it. However it is a very recently added feature (you'll need FLARE 26), so you might have to persuade him to invite EMC over for an upgrade session.

Tore Anderson

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