[clug] Is this logic correct? [12Tb RAID-5 array unlikely to rebuild]

[Adam Thomas]

2009/3/28 steve jenkin <sjenkin at canb.auug.org.au>:
> Anyone care to comment if this logic is correct or not?
> (Large drives in RAID-5 likely to be unable to rebuild)
> Seems simplistic to me...

I can't really comment on this better than Daniel Rutter (of Dan's
Data) does on his blog:

http://dansdata.blogsome.com/2008/10/23/death-of-raid-predicted-film-at-11/

>
> NetApp apparently spruik their double-parity RAID scheme as a solution.

Linux's MD system supports a double-parity RAID scheme under the name
of RAID6. See the mdadm documentation for details.

http://linux-raid.osdl.org/index.php/Main_Page

>
> eg:
> <http://www.usenix.org/events/fast04/tech/corbett/corbett_html/index.html>
> <http://storagemojo.com/2007/02/26/netapp-weighs-in-on-disks/>
>
> Links to FAST-08 papers:
> <http://storagemojo.com/2008/02/26/netapps-research-offensive/>
>
> ====================================
> <http://blogs.zdnet.com/storage/?p=162>
>
> Reads fail
> ----------
> SATA drives are commonly specified with an unrecoverable read error rate
> (URE) of 10^14. Which means that once every 100,000,000,000,000 bits,
> the disk will very politely tell you that, so sorry, but I really, truly
> can’t read that sector back to you.
>
> One hundred trillion bits is about 12 terabytes. Sound like a lot? Not
> in 2009.
>
> Disk capacities double
> -----------------------
> Disk drive capacities double every 18-24 months. We have 1 TB drives
> now, and in 2009 we’ll have 2 TB drives.
>
> With a 7 drive RAID 5 disk failure, you’ll have 6 remaining 2 TB drives.
> As the RAID controller is busily reading through those 6 disks to
> reconstruct the data from the failed drive, it is almost certain it will
> see an URE.
>
>
> --
> Steve Jenkin, Info Tech, Systems and Design Specialist.
> 0412 786 915 (+61 412 786 915)
> PO Box 48, Kippax ACT 2615, AUSTRALIA
>
> sjenkin at canb.auug.org.au http://members.tip.net.au/~sjenkin
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