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

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Message-ID: <4A94905F.7050705@redhat.com>
Date:	Tue, 25 Aug 2009 21:31:11 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	NeilBrown <neilb@...e.de>
CC:	Andrei Tanas <andrei@...as.ca>, linux-kernel@...r.kernel.org
Subject: Re: MD/RAID: what's wrong with sector 1953519935?

On 08/25/2009 09:24 PM, NeilBrown wrote:
> On Wed, August 26, 2009 11:06 am, Ric Wheeler wrote:
>> On 08/25/2009 08:50 PM, NeilBrown wrote:
>
>>> All 1TB drives are exactly the same size.
>>> If you create a single partition (e.g. sdb1) on such a device, and that
>>> partition starts at sector 63 (which is common), and create an md
>>> array using that partition, then the superblock will always be at the
>>> address you quote.
>>> The superblock is probably updated more often than any other block in
>>> the array, so there is probably an increased likelyhood of an error
>>> being reported against that sector.
>>>
>>> So it is not just a coincidence.
>>> Whether there is some deeper underlying problem though, I cannot say.
>>> Google only claims 68 matches for that number which doesn't seem
>>> big enough to be significant.
>>>
>>> NeilBrown
>>>
>>>
>>
>> Neil,
>>
>> 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,

ric




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