[Gllug] NFS problem

[Kostas Georgiou]

On Mon, Oct 22, 2007 at 08:46:26PM +0100, John Hearns wrote:

> On Mon, 2007-10-22 at 20:34 +0100, John Hearns wrote:
> > O
> > Sadly, in the era of Tbyte drives, the probability of dual failures
> > during reconstruction of RAID-5 sets is getting significant
> > 
> > http://www.eweek.com/article2/0%2C1895%2C2168821%2C00.asp
> > 
> > (see the answer to the last question)
> 
> I should make my reply a bit clearer here.
> Disks have increased hugely in capacity from the days when the original
> RAID concept was implemented. Let's say from 40gig disks to the 1Tbyte
> disks which are now common.
> However, the error rate stays the same (OK, I'll probably get some hard
> facts in reply to this, but it is broadly true).
> Reconstruction times for a failed RAID-5 set are longer with larger
> disks, and during that time you are more likely to hit a disk error,
> resulting in tears and storage engineers being sent to bed with no tea.

All[1] SATA disks have an uncorrectable bit error for every 10^14 bits
read, with 1TB disks close to 9*10^12 bits you have ~9% chance to get an
error when you read all the data. With 10x1TB disks in RAID5 you are more
or less certain that you'll get an error during reconstruction of the array.
Continuous checks in the background or even better RAID6 is needed when
you move to something of that size.

Cheers,
Kostas

[1] Actually I noticed recently one SATA disk (from WD I think) that claims a
10^15 error rate.
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