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Using fsck to defend against disk failures?

Using fsck to defend against disk failures?

Posted Jan 27, 2008 16:32 UTC (Sun) by nix (subscriber, #2304)
In reply to: Using fsck to defend against disk failures? by anton
Parent article: ext3 metaclustering

My mum's ancient 486 laptop had a really strange disk failure this 
Christmas. It started with a single bad sector, but then within about 
fifteen minutes one third of the sectors on the disk (in contiguous runs 
of varying length) were returning, not bad sectors, but `sector not 
found', i.e. the drive couldn't even find the sector address markers.

What I suspect may have happened, based on my extensive lack of experience 
in hard drive design, is that all the G forces the head assembly is 
exposed to whenever a seek happens had over time twisted the head reading 
the farthest side of whichever platter didn't contain the servo track out 
of true, so that when the servo track said it was over track X, the 
topmost heads were actually midway between tracks or something like that. 
In that position they couldn't read the sector addresses, couldn't find 
any data, and whoompfh, goodbye data.

(I've never heard of this failure mode anywhere else, and perhaps it was 
something different, but still, it was very strange. Disks *can* go mostly 
bad all at once. It's just rare.)


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Disk failures

Posted Jan 27, 2008 21:58 UTC (Sun) by anton (subscriber, #25547) [Link]

Disk drives have not used servo tracks for a long time, because one could no longer align all the heads precisely enough (e.g., because of thermal expansion). Instead, servo information exists on each platter, interspersed in some way with the data. I don't know when this change happened; a 15+-year old disk (486 generation) might still have a servo track. But couldn't the symptoms also be explained by the failure of just one of the heads?

Disk failures

Posted Jan 27, 2008 22:55 UTC (Sun) by nix (subscriber, #2304) [Link]

I said it was a prehistoric system, and indeed anything more modern than 
about, what, 1991 won't have this problem.

I'm not sure if a head failure could cause a failure to find sector 
address markers: I'm not sure if you could even distinguish the two cases 
without digging into the drive. (As I said, my expertise in hard drive 
engineering is notable mainly by its absence.)

It's just that heads are solid-state, and solid-state stuff doesn't die 
all that often, while the head assembly itself is being wrenched all over 
the place: simple bending could explain this, I think.


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