Re: [PATCH RESEND 2/2] SCSI: add scsi_device->retries

[Tejun Heo <htejun@gmail.com>]

Hello,

Kai Makisara wrote:
>> * st uses three retry limits - MAX_RETRIES, MAX_WRITE_RETRIES and
>>   MAX_READY_RETRIES, which are all zero.  This patch only converts
>>   MAX_RETRIES to sdev->retries.  Defining WRITE and READY retries in
>>   terms of sdev->retries would make more sense.
>>
> I am neither acking nor naking this now. The patch does not change st 
> behavior but moves part of the retry strategy out of the driver. (Which is 
> also good because it makes one of the retry limits user changeable.)
> 
> Combining all retry counts is something that may not be a good thing. 
> Below is justification why st has three different retry limits.
> 
> For some devices one number of retries is not perfect for all functions.
> 
> The firmware of tape devices usually retries quite thoroughly before 
> returning error. This is why the default zero applies to most cases.
> 
> MAX_WRITE_RETRIES is separate from MAX_RETRIES because a plausible 
> strategy might be (maybe not any more) to allow no retries for write but 
> allow retries for reading and repositioning. The writes should fail 
> directly so that the minimum number of marginal errors will be written. 
> The reads can be retried so that even marginal data can be recovered. 
> (Note that this may lead to tape positioning errors and may not be a 
> good strategy in all cases.)
> 
> MAX_READY_RETRIES is used for commands that don't move the tape. This is a 
> situation where retrying is probably harmless.

I see.  Then, how about adding and initializing STp->write_retries and 
STp->ready_retries according to SDp->retries (some reasonable default 
value, say write_retries is always zero while ready_retries is round up 
of retries * 1.1) and export both through sysfs?  That will give lower 
level primitive control as other ULDs do and also allow users to 
configure each timeout separately if necessary.

Thanks.

-- 
tejun