Re: [9fans] Recovering a venti from disk failure

[W B Hacker <wbh@conducive.org>]

erik quanstrom wrote:
>> Various studies seem to indicate failure rates are highly
>> correlated with drive model, vintage and manufacturer.
>> Assuming a RAID is built from similar disks, when one fails
>> the others are more likely to fail.
> 
> while it is true that some disks vintages are better than others, when
> one drive fails, the probability of the other drives failing has not
> changed.  this is the same as if you flip a coin ten times and get ten
> heads, the probability of flipping the same coin and getting heads, is
> still 1/2.
> 
>>> i think this corelation gives people the false impression that they do
>>> fail en masse, but that's really wrong.  the latent errors probablly
>>> happened months ago.
>> Yes but if there are many latent errors and/or the error rate
>> is going up it is time to replace it.
> 
> maybe.  the goggle paper you cited didn't find a strong correlation
> between smart errors (including block relocation) and failure.
> 
>> This is a good idea.  We did this in 1983, back when disks
>> were simpler beasts.  No RAID then of course.
> 
> even a better idea back then.  disks didn't have 1/4 million
> lines of firmware relocating blocks and doing other things to^w
> i mean for you.
> 
> - erik
> 
> 

And - lest we forget - a RAID array actually has a higher statistical chance of 
failure, and a *lower* MTBF than a single drive. Simple math.

What we gain is a reduced risk of *unrecoverable* damage, not fewer failures, 
per se.

Bill