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To: 9fans@cse.psu.edu
From: Joshua Wood <josh@utopian.net>
Subject: Re: [9fans] ata drive capabilities
Date: Wed, 26 Dec 2007 08:22:12 -0800
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> the google paper shows a 40% afr for the first 6 months after some
> smart errors appear.  (unfortunately they don't do numbers for
> a simple smart status.)

Yes, and I rather mischaracterized the google paper's comments on  
SMART. A reread (I first read them a few months ago) shows the above.  
Further, the CMU paper even references the google study on the SMART  
subject:

``They find that [ ... ] the value of several SMART counters  
correlate highly with failures.''

So SMART appears a little less dumb. I'd say meets the better than  
nothing criterion.

> from my understanding of how google do things, loosing a drive just
> means they need to replace it.  so it's cheeper to let drives fail.
> on the other hand, we have our main filesystem raided on an aoe
> appliance.  suppose that one of those raids has two disks showing
> a smart status of "will fail".  in this case i want to know the  
> elevated
> risk and i will allocate a spare drive to replace at least one of the
> drives.
>
> i guess this is the long way of saying, it all depends on how painful
> loosing your data might be.  if it's painful enough, even a poor tool
> like smart is better than nothing.
>
I agree (plus I was just wrong about SMART at first), though I do  
think your example above is about preventing downtime, not so much  
data loss (Even without smart entirely, and all the disks come up  
corrupt, we're all backed up within some acceptable window, right?)


> what a pity! it would have been so great to have had
> an objective assessment of reliability by manufacturer.
>
Since the CMU thing found no difference between disk *types*, I  
wonder if it might be that there's little difference between  
manufacturers either -- instead the difference is in manufacturing,  
i.e., `vintage' & the like.

> i've found it really quite hard to find useful data to
> indicate how reliable a drive might be.
>

I think Fig. 2, Sec. 4.2 of the CMU paper relates to that; the  
`infant mortality' of manufactured mechanical parts isn't captured in  
MTTF -- but IDEMA is apparently going to solve this by replacing the  
single MTTF number that I don't quite understand with 4 different  
MTTF numbers, one for each `phase' of a disk's life.

--
Josh