linux-kernel - Re: [PATCH] scsi_lib.c: continue after MEDIUM

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Date:	Wed, 31 Jan 2007 09:36:53 -0500
From:	Ric Wheeler <ric@....com>
To:	Jeff Garzik <jeff@...zik.org>
CC:	Mark Lord <liml@....ca>, "Eric D. Mudama" <edmudama@...il.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	linux-kernel@...r.kernel.org,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: [PATCH] scsi_lib.c: continue after MEDIUM_ERROR



Jeff Garzik wrote:
> Mark Lord wrote:
>> Eric D. Mudama wrote:
>>>
>>> Actually, it's possibly worse, since each failure in libata will 
>>> generate 3-4 retries.  With existing ATA error recovery in the 
>>> drives, that's about 3 seconds per retry on average, or 12 seconds 
>>> per failure.  Multiply that by the number of blocks past the error to 
>>> complete the request..
>>
>> It really beats the alternative of a forced reboot
>> due to, say, superblock I/O failing because it happened
>> to get merged with an unrelated I/O which then failed..
>> Etc..
> 
> 
> FWIW -- speaking generally -- I think there are inevitable areas where 
> libata error handling combined with SCSI error handling results in 
> suboptimal error handling.
> 
> Just creating a list of "<this behavior> should be handled <this way>, 
> but in reality is handled in <this silly way>" would be very helpful.

I agree - Tejun has done a great job at giving us a great base. Next step is to 
get clarity on what the types of errors are and how to differentiate between 
them (and maybe how that would change by class of device?).

> 
> Error handling is tough to get right, because the code is exercised so 
> infrequently.  Tejun has actually done an above-average job here, by 
> making device probe, hotplug and other "exceptions" go through the 
> libata EH code, thereby exercising the EH code more than one might 
> normally assume.
> 
> Some errors in libata probably should not be retried more than once, 
> when we have a definitive diagnosis.  Suggestions for improvements are 
> welcome.
> 
>     Jeff

One thing that we find really useful is to inject real errors into devices. Mark 
has some patches that let us inject media errors, we also bring back failed 
drives and run them through testing and occasionally get to use analyzers, etc 
to inject odd ball errors.

Hopefully, we will get some time to brainstorm about this at the workshop,

ric
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