[ILUG] Why RAID

[Paul Jakma]

On Tue, 13 Jul 2004, John P. Looney wrote:

> 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.

Ouch. RAID1+0 on four disks, so you get capacity and speed of 
two-disk RAID0 but reliability of four disks where you can only 
tolerate one failure? You'd be better off with RAID5 on three disks + 
hot spare - sameish read performance, same capacity, but you can 
tolerate two failures (provided second failure doesnt occur before 
hot spare has synced).

Or RAID-5 on all four disks, probably better performance - same 
reliability.

Or try RAID-6, n-2 reliability. (but new)

> 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

Yuk yes. Dont *ever* use the "RAID" features of these things, dont 
*ever* let yourself be sucked into using their own drivers. On the 
other hand, I believe it's possible for Promise (iirc) to setup 
device-mapper mirror to grok their on-disk RAID format - bit of a 
manual process at moment, hopefully in future there'll be tools to 
automate it all. So you can use their BIOS RAID to boot, but use sane 
linux drivers after that.

> You can then get little hotswap drive cages for SCSI (not seen it 
> for SATA yet),

You can, I have a cage here. See www.acme-technologies.co.uk, Encom 
resell them.

> 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.

Ouch :(

Hardware RAID controllers sometimes do funny things, eg I've had 
problems with Mylex controllers refusing to recognise a disk anymore 
that had been pulled as good. The stubborn thing insisted the disk 
had died, and by golly it wasnt going to recognise it anymore. I had 
to change the SCSI ID to make it think it was a new disk - luckily 
the drive was *not* in a cage (where IDs usually are assigned by the 
cage). There was no other apparent way to make it work.

> john

regards,
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