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

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Date:	Wed, 26 Aug 2009 11:24:05 +1000 (EST)
From:	"NeilBrown" <neilb@...e.de>
To:	"Ric Wheeler" <rwheeler@...hat.com>
Cc:	"Andrei Tanas" <andrei@...as.ca>, linux-kernel@...r.kernel.org
Subject: Re: MD/RAID: what's wrong with sector 1953519935?

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.... :-(

>
> 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

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