How to create a terabyte storage array?

[David Hart]

On Wed, 2005-11-30 at 10:33 -0500, Zach wrote:
> Assuming reliable backups are being performed (they should be), then
> what raid really buys you, aside from performance, is minimized
> downtime by saving you the trouble of restoring from backup.

For the sake of argument, let's assume a disk will fail on average in
five years (a reasonable starting point, I think).  If you put five
disks into one LVM volume you can expect that volume to fail once a
year. (Let's not go into the 'bathtub curve' at the moment.)

If you put those same disks into a raid5 (assuming you replace failing
disks promptly) you'll _increase_ the reliability of your volume by a
factor of five.

In other words, a single (5 disk) LVM volume is about 25 times more
likely to fail than the same disks in a raid5.

You'll lose 20% of your storage space to the raid parity, but that IMO
is a small price to pay for the much increased reliability.

> For a family file server, downtime in the event of disk failure may be
> acceptable.  If this is the case, LVM may be a simpler approach, and
> combined with regular backups, is very suitable.
> 
> Raid won't save you from data corruption not related to disk failure.
> And since the file server is being accessed on the lan, the
> performance bottleneck is likely the network itself, especially if
> multiple people are working with lots of data simultaneously, like
> large video files, etc.

Neither method will save you from data corruption not related to disk
failure so it's irrelevant.

-- 
David Hart <ubuntu at tonix.org>