[ale] If your using large Sata drives in raid 5/6 ....

[Jeff Hubbs]

This is why I steered away from RAID5 and 6 for the file server I built 
for my employer.  I intended to have a "big slow" SATA array and a 
"small fast" SAS array - I planned to use 1+0 for the latter and 5 for 
the former, but when I began looking into this issue I got skittish and 
decided to go 1+0 for the big slow array as well.

On 2/2/10 5:40 PM, Greg Freemyer wrote:
> All,
>
> I think the below is accurate, but cmiiw.
>
> ===
> If your using normal big drives (1TB, etc.) in a raid-5 array, the
> general consensus of the mdraid list is your crazy.  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.
>
> 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.
>
> 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.
>
> See<http://www.anandtech.com/storage/showdoc.aspx?i=3691&p=1>
>
> The whole article is good, but this paragraph is what really got my attention:
>
> "From a numbers perspective, Western Digital estimates that the use of
> 4K sectors will give them an immediate 7%-11% increase in format
> efficiency. ECC burst error correction stands to improve by 50%, and
> the overall error rate capability improves by 2 orders of magnitude.
> In theory these reliability benefits should immediately apply to all
> 4K sector drives (making the Advanced Format drives more reliable than
> regular drives), but Western Digital is not pushing that idea at this
> time."
>
> So maybe raid-5 will once again be a reasonable choice again in the
> future.  (I think these drives may already be available at least as
> engineering samples.)
>
> Greg
>