Date:	Mon, 31 Aug 2009 08:04:26 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Andrei Tanas <andrei@...as.ca>, NeilBrown <neilb@...e.de>,
	linux-kernel@...r.kernel.org,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	linux-scsi@...r.kernel.org, Jeff Garzik <jgarzik@...hat.com>,
	Mark Lord <mlord@...ox.com>
Subject: Re: MD/RAID time out writing superblock

On 08/31/2009 04:10 AM, Tejun Heo wrote:
> Ric Wheeler wrote:
>    
>> On 08/27/2009 05:22 PM, Andrei Tanas wrote:
>>      
>>> Hello,
>>>
>>> This is about the same problem that I wrote two days ago (md gets an
>>> error
>>> while writing superblock and fails a hard drive).
>>>
>>> I've tried to figure out what's really going on, and as far as I can
>>> tell,
>>> the disk doesn't really fail (as confirmed by multiple tests), it
>>> times out
>>> trying to execute ATA_CMD_FLUSH_EXT ("at2.00 cmd ea..." in the log)
>>> command. The reason for this I believe is that md_super_write queues the
>>> write comand with BIO_RW_SYNCIO flag.
>>> As I wrote before, with 32MB cache it is conceivable that it will take
>>> the
>>> drive longer than 30 seconds (defined by SD_TIMEOUT in scsi/sd.h) to
>>> flush
>>> its buffers.
>>>
>>> Changing safe_mode_delay to more conservative 2 seconds should definitely
>>> help, but is it really necessary to write the superblock synchronously
>>> when
>>> array changes status from active to active-idle?
>>>
>>> [90307.328266] ata2.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x6
>>> frozen
>>> [90307.328275] ata2.00: cmd ea/00:00:00:00:00/00:00:00:00:00/a0 tag 0
>>> [90307.328277]          res 40/00:01:01:4f:c2/00:00:00:00:00/00 Emask 0x4
>>> (timeout)
>>> [90307.328280] ata2.00: status: { DRDY }
>>> [90307.328288] ata2: hard resetting link
>>> [90313.218511] ata2: link is slow to respond, please be patient (ready=0)
>>> [90317.377711] ata2: SRST failed (errno=-16)
>>> [90317.377720] ata2: hard resetting link
>>> [90318.251720] ata2: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
>>> [90318.338026] ata2.00: configured for UDMA/133
>>> [90318.338062] ata2: EH complete
>>> [90318.370625] end_request: I/O error, dev sdb, sector 1953519935
>>> [90318.370632] md: super_written gets error=-5, uptodate=0
>>>
>>>
>>>        
>> 30 seconds is a very long time for a drive to respond, but I think that
>> your explanation fits the facts pretty well...
>>      
> Even with 32MB cache, 30secs should be more than enough.  It's not
> like the drive is gonna do random write on those.  It's likely to make
> only very few number of strokes over the platter and it really
> shouldn't take very long.  I'm yet to see an actual case where a
> properly functioning drive timed out flush because the flush itself
> took long enough.
>
>    

I agree - vendors put a lot of pressure on drive manufacturers to finish 
up (even during error recovery) in much less than 30 seconds. The push 
was always for something closer to 15 seconds iirc.

>> The drive might take a longer time like this when doing error handling
>> (sector remapping, etc), but then I would expect to see your remapped
>> sector count grow.
>>      
> Yes, this is a possibility and according to the spec, libata EH should
> be retrying flushes a few times before giving up but I'm not sure
> whether keeping retrying for several minutes is a good idea either.
> Is it?
>
> Thanks.
>
>    

I don't think that retrying for minutes is a good idea. I wonder if this 
could be caused by power issues or cable issues to the drive?

Ric

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