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

[Hannes Reinecke]

This is a multi-part message in MIME format.Hannes Reinecke wrote:
> Sergei Shtylyov wrote:
>> Hello.
>>
>> Hannes Reinecke wrote:
>>
>>> Actually, I think we have two separate issues here:
>>> 1) The need of having more detailed I/O errors even in the fs layer. This
>>>    we've already discussed at the LSF, consensus here is to allow other
>>>    errors than just 'EIO'.
>>>    Instead of Mike's approach I would rather use existing error codes
>>> here;
>>>    this will make the transition somewhat easier.
>>>    Initially I would propose to return 'ENOLINK' for a transport failure,
>>>    'EIO' for a non-retryable failure on the target, and 'ENODEV' for a
>>>    retryable failure on the target.
>>    Are you sure it's not vice versa: EIO for retryable and ENODEV for
>> non-retryable failures. ENODEV looks more like permanent condition to me.
>>
> Ok, can do.
> And looking a the error numbers again, maybe we should be using 'EREMOTEIO'
> for non-retryable failures.
> 
> So we would be ending with:
> 
> ENOLINK: transport failure
> EIO: retryable remote failure
> EREMOTEIO: non-retryable remote failure
> 
And here is the corresponding patch.
Compile tested only; just to give an idea of the possible implementation.

I have decided to pass the I/O failure information in-line:
- scsi_check_sense() might now return 'TARGET_ERROR' to signal
  a permanent error
- scsi_decide_disposition() sets the driver byte of the result
  field to 'DID_TARGET_FAILURE' if a return code of 'TARGET_ERROR'
  is encountered.
- scsi_io_completion() sets the error to ENOLINK for DID_TRANSPORT_FAILFAST,
  EREMOTEIO for DID_TARGET_FAILURE, and EIO for any other error. It also
  resets DID_TARGET_FAILURE back to DID_OK once the error code is set.

I'm not 100% happy with this patch; DID_TARGET_FAILURE is really just
a communication vehicle to signal the permanent target failure.
I looked at passing this information directly via an explicit argument
to scsi_finish_command(), but this would include changing
scsi_io_completion(), too. As both of them are exported / public
interfaces I didn't like modifying them.

Another possibility would be to re-use / redefine the 'DRIVER_'
bits; they don't seem to be used a the moment. Eg 'DRIVER_HARD'
for permanent errors, DRIVER_SOFT for link failures.

Opinions welcome.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
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