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Go Back   Linux Archive > Redhat > Device-mapper Development

 
 
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Old 05-22-2012, 12:54 PM
Mike Snitzer
 
Default scsi: Fix dm-multipath starvation when scsi host is busy

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>

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Old 05-22-2012, 01:01 PM
Hannes Reinecke
 
Default 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 <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
hare@suse.de +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)

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