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[dm-devel] Re: mpath: don't fail paths on first error



Hi Mike,

Mike Christie wrote:
The problem we see a lot at Red Hat is that if drivers fail a command with DID_BUS_BUSY or DID_ERROR for something like underrun or even for transient path problems, we can normally recover from this pretty quickly and we do not need to switch path groups.

Yeah, I thought about this, too.
queue_if_no_path/no_path_retry will prevent IO from being fail upwards, but just switching paths can cause a lot of strain on the target, so we might want to prevent path switching when we do not need to. If we are using a box that requires manual failover or a box that does not use manual failover but still has to shift resources between storage controllers when switching paths, we most likely do not want to mark paths failed for these transient errors.

Well, the original design idea was that it always will be quicker or
less error-prone to just move the I/O to the next path.
Seeing that this is not always the case this approach is probably
better.

The attached patch allows us to wait X seconds before marking a path as failed. If within X seconds from seeing the first IO error, we do not see a IO complete successfully then we mark a path as failed. This patch work best with the fail fast enhancements ones where for a lot of path problems the fast io fail / recovery timeout will fail io quickly to us and the test IOs do not get stuck, and where some errors like DID_ERROR are not even failed fast.

The patch should apply over linus's tree or scsi-misc.

Thanks for this, Mike.

Signed-off-by: Hannes Reinecke <hare suse de>

Cheers,

Hannes
--
Dr. Hannes Reinecke		      zSeries & Storage
hare suse de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: Markus Rex, HRB 16746 (AG Nürnberg)


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