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Re: [dm-devel] [RFC] training mpath to discern between SCSI errors



Hi Hannes,

(10/18/10 20:55), Hannes Reinecke wrote:
>> Does 'retryable' of EIO mean retryable in multipath layer?
>> If so, what is the difference between EIO and ENOLINK?
>>
> Yes, EIO is intended for errors which should be retried at the
> multipath layer. This does _not_ include transport errors, which are
> signalled by ENOLINK.
> 
> Basically, ENOLINK is a transport error, and EIO just means
> something is wrong and we weren't able to classify it properly.
> If we were, it'd be either ENOLINK or EREMOTEIO.
> 
>> I've heard of a case where just retrying within path-group is
>> preferred to (relatively costly) switching group.
>> So, if EIO (or other error code) can be used to indicate such type
>> of errors, it's nice.
>>
> Yes, that was one of the intention.

Great to hear that.

And when it comes to retrying, the next problem is who controls it.
I don't think it's good to duplicate retry logic in multipath and
underlying device like SCSI (i.e. sd retries 5 times).
So perhaps we need a way to disable (or limit) retries in underlying
device at least.

>> Also (although this might be a bit off topic from your patch),
>> can we expand such a distinction to what should be logged?
>> Currently, it's difficult to distinguish important SCSI/block errors
>> and less important ones in kernel log.
>> For example, when I get a link failure on sda, kernel prints something
>> like below, regardless of whether the I/O is recovered by multipathing or not:
>>   end_request: I/O error, dev sda, sector XXXXX
>>
> Indeed, when using the above we could be modifying the above
> message, eg by
> 
> end_request: transport error, dev sda, sector XXXXX
> 
> or
> 
> end_request: target error, dev sda, sector XXXXX
> 
> which would improve the output noticeable.

It improves but still they look like critical errors
even if multipath saves them.

When I see this:

  end_request: target error, dev sda, sector XXXXX

I can't tell whether it's a real error visible to user space
or it's just recoverred by multipath retry/failover afterwards.


>> Setting REQ_QUIET in dm-multipath could mask the message
>> but also other important ones in SCSI.
>>
> Hmm. Not sure about that, but I think the above modifications will
> be useful already.
> 
> I'll be sending an updated patch.

Thank you. I'm looking for that.

-- 
Jun'ichi Nomura, NEC Corporation


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