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Go Back   Linux Archive > Redhat > Device-mapper Development

 
 
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Old 06-05-2008, 05:25 PM
Mike Christie
 
Default mpath: don't fail paths on first error

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.


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.


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.
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Old 06-06-2008, 02:18 PM
Hannes Reinecke
 
Default 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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