Raid 5

Andrew McNabb amcnabb at mcnabbs.org
Thu Dec 1 17:54:39 MST 2005


On Thu, Dec 01, 2005 at 05:15:08PM -0700, Matthew Ross Walker wrote:
> Since then, RAID just hasn't seemed that great to me. To make it
> effective as disaster prevention, you have to go with RAID1, or 0+1,
> and you have to have a significant number of drives in the array.
> 
> If you're just looking for greater storage space, and loss of data isn't
> an issue, definately go with RAID0.

Like many people, you used to have the impression that RAID is something
that it isn't.  Sometimes people think that RAID is some sort of backup
That way it would be so easy.

Losing 3 disks at once in an array is much less common than losing 1
disk, but you should still prepare for both cases.  In a good setup, you
can recover from 1 failed disk in barely more time than it takes to get
a disk.  Also in a good setup, you can recover from 3 failed disks in
the amount of time it takes to get 3 new disks plus the amount of time
it takes to restore your backups.

Anyway, RAID-0 is a very bad idea because it will fail much more
frequently than a single disk.

Suppose you have a disk with a 1/1000 chance of dying on any given day,
which I think is a reasonable off-the-cuff estimate based on my
experience.  Then, on any given day, you have a 1/1000 chance of losing
all of your data (and have to go to your backups).  Now suppose I take 4
of these and put them together into RAID-0.  Now I have nearly a 1/250
chance of losing all of my data on any given day.  However, if I had put
those 4 disks together, the odds of losing everything would be 1/166667.

You still have to worry about the fact that drives can die at the same
time due to power problems, but picking a good power supply can make a
huge difference with these.  And there's not too much you can do about
nature.

If you use RAID for what it is, and not for something else, it's a great
help.

-- 
Andrew McNabb
http://www.mcnabbs.org/andrew/
PGP Fingerprint: 8A17 B57C 6879 1863 DE55  8012 AB4D 6098 8826 6868
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