linux-kernel - Re: MD/RAID: what's wrong with sector 1953519935?

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Message-ID: <4A94A0E6.4020401@redhat.com>
Date:	Tue, 25 Aug 2009 22:41:42 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Andrei Tanas <andrei@...as.ca>
CC:	"'NeilBrown'" <neilb@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: MD/RAID: what's wrong with sector 1953519935?

On 08/25/2009 10:22 PM, Andrei Tanas wrote:
>>>> One thing that can happen is when we have a hot spot (like the super
>>>> block) on high capacity drives is that the frequent write degrade
>> the
>>>> data in adjacent tracks.  Some drives have firmware that watches for
>>>> this and rewrites adjacent tracks, but it is also a good idea to
>> avoid
>>>> too frequent updates.
>>>
>>> Yet another detail to worry about.... :-(
>>
>> it never ends :-)
>>
>>>
>>>>
>>>> Didn't you have a tunable to decrease this update frequency?
>>>
>>> /sys/block/mdX/md/safe_mode_delay
>>> is a time in seconds (Default 0.200) between when the last write to
>>> the array completes and when the superblock is marked as clean.
>>> Depending on the actual rate of writes to the array, the superblock
>>> can be updated as much as twice in this time (once to mark dirty,
>>> once to mark clean).
>>>
>>> Increasing the number can decrease the update frequency of the
>> superblock,
>>> but the exact effect on update frequency is very load-dependant.
>>>
>>> Obviously a write-intent-bitmap, which is rarely more that a few
>>> sectors, can also see lots of updates, and it is harder to tune
>>> that (you have to set things up when you create the bitmap).
>>>
>>> NeilBrown
>>>
>>
>> We did see issues in practice with adjacent sectors with some drives,
>> so this
>> one is worth tuning down.
>>
>> I would suggest that Andrei might try to write and clear the IO error
>> at that
>> offset. You can use Mark Lord's hdparm to clear a specific sector or
>> just do the
>> math (carefully!) and dd over it. It the write succeeds (without
>> bumping your
>> remapped sectors count) this is a likely match to this problem,
>
> I've tried dd multiple times, it always succeeds, and the relocated sector
> count is currently 1 on this drive, even though this particular fault
> happened at least 3 times so far.
>

I would bump that count way up (say to 2) and see if you have an issue...

ric

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