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

[Matt Rideout]

This past weekend, I migrated my filer from 3-disk RAID 5 to 4-disk RAID 
10 primarily for this reason. Everything that was of much value was 
backed up elsewhere, so the basic procedure for pulling this off with 5 
disks instead of 7 was:

1. Create the RAID 10 array with 2 missing members
2. rsync
3. Remove 2 members from the RAID 5 array, add them into the RAID 10 
array and let the array rebuild

The performance boost is very noticeable, especially if I'm working on 
the ESX box that this filer feeds when the nightly rdiff-backup jobs 
kick in.

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
>