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

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Date:	Wed, 31 Jan 2007 10:08:12 -0500
From:	Mark Lord <liml@....ca>
To:	Ric Wheeler <ric@....com>
Cc:	"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>, dougg@...que.net
Subject: Re: [PATCH] scsi_lib.c: continue after MEDIUM_ERROR

Ric Wheeler wrote:
> Mark Lord wrote:
>> Eric D. Mudama wrote:
>>> Actually, it's possibly worse, since each failure in libata will 
>>> generate 3-4 retries. 

(note: libata does *not* generate retries for medium errors;
 the looping is driven by the SCSI mid-layer code).

>> 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..
>>
>> Definitely an improvement.
>>
>> The number of retries is an entirely separate issue.
>> If we really care about it, then we should fix SD_MAX_RETRIES.
>>
>> The current value of 5 is *way* too high.  It should be zero or one.
..
> I think that drives retry enough, we should leave retry at zero for 
> normal (non-removable) drives. Should this  be a policy we can set like 
> we do with NCQ queue depth via /sys ?

Or perhaps we could have the mid-layer always "early-exit"
without retries for "MEDIUM_ERROR", and still do retries for the rest.

When libata reports a MEDIUM_ERROR to us, we *know* it's non-recoverable,
as the drive itself has already done internal retries (libata uses the
"with retry" ATA opcodes for this).

But meanwhile, we still have the original issue too, where a single stray
bad sector can blow a system out of the water, because the mid-layer
currently aborts everything after it from a large merged request.

Thus the original patch from this thread.  :)

Cheers
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