Scaremongering, or legitimate things to worry about? Lots of the "Talkback"
comments are that ZDNet is over the top these days, but it seems to me he's got
some legitimate points.
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Why RAID 5 stops working in 2009 | Storage Bits | ZDNet.com
Disks fail
While disks are incredibly reliable devices, they do fail. Our best data -
from CMU and Google - finds that over 3% of drives fail each year in the
first three years of drive life, and then failure rates start rising fast.
With 7 brand new disks, you have ~20% chance of seeing a disk failure each
year. Factor in the rising failure rate with age and over 4 years you are
almost certain to see a disk failure during the life of those disks.
But you´re protected by RAID 5, right? Not in 2009.
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.
So the read fails. And when that happens, you are one unhappy camper. The
message "we can´t read this RAID volume" travels up the chain of command
until an error message is presented on the screen. 12 TB of your carefully
protected - you thought! - data is gone. Oh, you didn´t back it up to
tape? Bummer!
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More here with links: http://blogs.zdnet.com/storage/?p=162
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