David Dillow, on 10/24/2009 02:08 AM wrote:
On Fri, 2009-10-23 at 12:50 -0400, Vu Pham wrote:
David Dillow wrote:
I don't know much about multipath in ALUA mode.
How would multipath driver (in ALUA mode) to switch path? (ie. basing on
what criteria?)
ALUA sets the priority of each path, and generally multipath is set to
round-robin among all paths of the same priority. So, paths going to the
primary controller of a LUN get the best priority and are used
preferentially over the backup paths. Once no more paths in a priority
group are active, the round-robin selector will fall back to the next
highest priority path group.
The multipath core will immediately fail a path if the lower layers
propagate an error up to it, such that an I/O request completes in
error. If it has failed the path, it will start sending requests down
alternate paths without waiting for the queue to drain on the first one.
ALUA is not like RDAC -- in ALUA, all paths are valid to use, but some
paths will give better performance. You do not necessarily need to give
the array a command to move the LUN to another controller, so there's no
reason to wait for a queue to drain.
At least that is the way I understand things, having picked my way
through the block layer, multipath core, and device handlers.
Unfortunately, it isn't always so simple. Sometimes there are
dependencies between order of requests, so you have to drain the failed
path before sending retries to the alternate path to make sure that the
retries will not interfere with not yet aborted requests from the failed
paths.
Just to make things more precise.
Vlad
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