[plug] RAID, and what it can't solve

[Scott Middleton]

I totally agree. I have heard often the last couple of years "but i have
RAID", I always say "What is it about RAID that you thought it was safe"

Realistically; If the chance of a HDD drive failing increases
exponentially over time, then what are the chances they will fail if you
have 5 HDDs. Statistically you are 5 times more likely to have the spare
one fail than a person with a single HDD. When that fails you are 4
times more likely... 

RAID is for Redundancy and speed not safety. It is designed to keep your
data centre on-line during critical times. Then at your leisure
rebuilding the array. 

As Craig noted about a double disk failure. In my experience, that
happens often. So if you are going to do RAID have at least 2 spares.
Probably three HDD will fail then :)

We just finished installing Linux on a computer with 12 SATA 240GB HDD
on an escalade 9500 PCI-X with Hot Swap Caddies. One hot spare and RAID
5 the client has a total disk space of 2.27TB. Takes a long time to
format :) This computer can afford to lose 2 HDDs but not at the same
time. One can fail and once the array has finished rebuilding the other
can fail. If 2 HDD fail the then Array is degraded and Data loss is
likely. Load testing this was a bitch since it took hours to rebuild the
array every time we purposefully flattened it.


On Tue, 2004-06-22 at 07:08, Craig Ringer wrote:
> Hi folks
> 
> I thought I'd drop a note to remind anyone considering RAID that it's
> not total data protection. Backups, backups, backups.
> 
> Why do I mention this now? Because I just had my own paranoia borne out
> by a double disk failure in a 4 disk RAID 5 array. Darn thing was half
> way through rebuilding onto the hot spare when the second disk died. The
> array is only two days old, too - 4x250GB SATA Western Digital disks, +
> hot spare.
> 
> Thankfully, I keep system snapshots on SATA disks (mmm... hot swap) in
> addition to tape backups, and of course I'd just updated the snapshot
> when migrating from the old array. I was able to make a fresh snapshot
> off the degraded array before trying to rebuild - just in case something
> might've been misssed - so didn't lose even a tiny bit of data.
> 
> --
> Craig Ringer
> 
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-- 
Scott Middleton <scott at linuxit.com.au>
Linux Information Technology