[ILUG] Why RAID

[John P. Looney]

On Tue, Jul 13, 2004 at 12:32:18PM +0100, Timothy Murphy mentioned:
> Reading about all the problems with RAID,
> I wonder why people bother with it?
> Is it worth the hassle?

 If you leave your cheap IDE disk on 24 hours a day, you have a one in ten
chance of losing a disk after the first year, a two in ten chance of
losing it after the second year. I assume that will tell you how important
it is to have RAID at all. 

 Other benefits of RAID are speed. If you have two disks, in RAID1, you
get twice the read performance, and a little over half the write
performance (you can choose which disk to read from, but have to write to
both). As 80% of the time you are reading, that's a plus. You can also
configure them in RAID1+0, which means you get two or three pairs of
disks, and mirror each pair, so that you get two or three "reliable"
disks. You can then stripe them, so you can get three times the read and
three times the write speed. Taken together, your six-disk set is now 600%
faster at reading, and (50%*300%) 150% faster at writing.

 The problem is that you can cut corners. At the bottom end, you can use
software RAID. It's free, it's often faster than hardware RAID, but it's
tricky to get working and when you lose a disk, getting a bootable system
going can be time consuming. The next level up are the crappy pretend
hardware RAID. Highpoint and Promise come to mind here. This can be a
world of pain. Promise drivers are poor, and the machine still falls over
when a disk dies - it has to be replaced with a new one before it'll work.
If you are lucky - some cheap raid cards keep going, and unless you are
watching your syslog messages, you won't know you've lost a disk...until
the second one dies.

 You can then get a proper hardware RAID card. Unless you spend big money,
it'll be slower than software RAID, but it takes care of everything for
you. You can put 12 disks in a box, and make them all look like they are
one big happy disk. If you lose a disk, it beeps and lights LEDs for you.
This is invaluable. 

 You can then get little hotswap drive cages for SCSI (not seen it for
SATA yet), so you can swap disks back in without losing your machine.
Though this can also be painful - we learned the hardway that if you put a
certain brand of IBM disk into an Intel SR2300 without upgrading the
disk's firmware, you can corrupt your RAID array.

john