Re: [dm-devel] [PATCH v2] scsi: Fix dm-multipath starvation when scsi host is busy

[Hannes Reinecke <hare suse de>]

On 05/22/2012 02:54 PM, Mike Snitzer wrote:
> On Tue, May 22 2012 at  5:57am -0400,
> Jun'ichi Nomura <j-nomura ce jp nec com> 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:
>> http://www.redhat.com/archives/dm-devel/2012-May/msg00123.html
>>
>> 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 <bernd schubert itwm fraunhofer de>
>> Tested-by: Bernd Schubert <bernd schubert itwm fraunhofer de>
>> Signed-off-by: Jun'ichi Nomura <j-nomura ce jp nec com>
>> Cc: James Bottomley <James Bottomley HansenPartnership com>
>> Cc: Mike Snitzer <snitzer redhat com>
> 
> Acked-by: Mike Snitzer <snitzer redhat com>
> 
Acked-by: Hannes Reinecke <hare suse de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
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