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

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Date:	Tue, 25 Aug 2009 22:22:17 -0400
From:	"Andrei Tanas" <andrei@...as.ca>
To:	"'Ric Wheeler'" <rwheeler@...hat.com>,
	"'NeilBrown'" <neilb@...e.de>
Cc:	<linux-kernel@...r.kernel.org>
Subject: RE: MD/RAID: what's wrong with sector 1953519935?

> >> 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.

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