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

steve jenkin sjenkin at canb.auug.org.au
Sat Mar 28 08:22:05 GMT 2009

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...

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


Links to FAST-08 papers:


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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