scsi: Fix dm-multipath starvation when scsi host is busy
On 05/22/2012 02:54 PM, Mike Snitzer wrote:
> On Tue, May 22 2012 at 5:57am -0400,
> Jun'ichi Nomura <firstname.lastname@example.org> wrote:
>> On 05/22/12 18:13, James Bottomley wrote:
>>> Isn't a more understandable explanation:
>> Thank you. That's quite elegant.
>> I replaced the description with it.
>>> And please put a comment in the code as well otherwise someone will
>>> eventually send a "fix" for this because we're not paying attention to
>>> host busy (and I'll have forgotten about the issue by then and might
>>> apply it).
>> Added the comment in code.
>>> A final note is that this is more a band aid than a fix because this is
>>> still a congestion situation dm-mp should be aware of.
>> Yes. To do that, we have to generalize the concept of "host"
>> and share it with block layer.
>> Attached below is the revised patch.
>> [PATCH v2] scsi: Fix dm-multipath starvation when scsi host is busy
>> block congestion control doesn't have any concept of fairness across
>> multiple queues. This means that if SCSI reports the host as busy in
>> the queue congestion control it can result in an unfair starvation
>> situation in dm-mp if there are multiple multipath devices on the same
>> host. For example:
>> The fix for this is to report only the sdev busy state (and ignore the
>> host busy state) in the block congestion control call back.
>> The host is still congested, but the SCSI subsystem will sort out the
>> congestion in a fair way because it knows the relation between the
>> queues and the host.
>> Reported-by: Bernd Schubert <email@example.com>
>> Tested-by: Bernd Schubert <firstname.lastname@example.org>
>> Signed-off-by: Jun'ichi Nomura <email@example.com>
>> Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
>> Cc: Mike Snitzer <firstname.lastname@example.org>
> Acked-by: Mike Snitzer <email@example.com>
Acked-by: Hannes Reinecke <firstname.lastname@example.org>
Dr. Hannes Reinecke zSeries & Storage
email@example.com +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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