Re: If your using large Sata drives in raid 5/6 ....

[John Robinson <john.robinson@anonymous.org.uk>]

On 02/02/2010 22:46, Greg Freemyer wrote:
> All,
> 
> I think the below is accurate, but please cmiiw or misunderstand.
> 
> ===
> If your using normal big drives (1TB, etc.) in a raid-5 array, the
> general consensus of this list is that it is a bad idea.  The reason being
> that the sector error rate for a bad sector has not changed with
> increasing density.
> 
> So in the days of 1GB drives, the likelihood of a undetected /
> repaired bad sector was actually pretty low for the drive as whole.
> But for today's 1TB drives, the odds are 1000x worse.  ie. 1000x more
> sectors with the same basic failure rate per sector.
> 
> So a raid-5 composed of 1TB drives is 1000x more likely to be unable
> to rebuild itself after a drive failure than a raid-5 built from 1 GB
> drives of yesteryear.  Thus the current recommendation is to use raid
> 6 with high density drives.

That sounds about right. One might still see RAID-5 as a way of pushing 
data loss through bad sectors back into a comfortable zone. After all, 
the likelihood of the same sector going bad on one of the other drives 
should be relatively small. Unfortunately it's too long since I studied 
probability for me to work it out properly. Then, to also protect 
yourself against dead drives, adding another drive a la RAID-6 sounds 
like the answer. But you can't think of RAID-6 protecting you from 2 
drive failures any more.

What is more, you need Linux md's implementation of single-sector 
recovery/rewriting for this to work. You cannot go around failing arrays 
because occasional single-sector reads fail.

> The good news is that Western Digital is apparently introducing a new
> series of drives with an error rate "2 orders of magnitude" better
> than the current generation.

It's not borne out in their figures; WD quote "less than 1 in 10^15 
bits" which is the same as they quote for their older drives.

What sums I've done, on the basis of a 1 in 10^15 bit unrecoverable 
error rate, suggest you've a 1 in 63 chance of getting an uncorrectable 
error while reading the whole surface of their 2TB disc. Read the whole 
disc 44 times and you've a 50/50 chance of hitting an uncorrectable error.

You could read the whole drive in about 5 hours, according to the spec 
(at 110MB/s), so if you keep your drive busy you're going to reach this 
point in about 9 days. If you had a 5-drive array, you're going to get 
here inside 2 days.

Bear in mind that this is on a drive working perfectly correctly as 
specified. We have to expect to be recovering from failed reads daily.

</doom> ;-)

Cheers,

John.

PS. Wish I'd written down my working for this.
PPS. I'm not having a go at WD; other manufacturers' specs are similar.