booting from raid

[blue&whiteman]

I have a wd 120gb 8mb cache and would like to get another. will the built in raid capability in disk utility allow me to create a raid that I can also boot from? obviously I will need an ata 133 pci card as a raid on my logic boards built in ata 33 will be limited.

i'm new to raid so any advice would be great. I simply want to raid 2 identical drives and be able to boot from them. what type raid and other things should I do for max performance and stability?

[Pengu]

I don't think Apple's software RAID allows booting. I think SoftRAID does though. Why not buy an ATA-133 RAID pci controller. Not much extra cash last time i checked.

[scruffy]

I believe you can, but the GUI doesn't make it obvious - it's sort of undocumented, so make sure you back up anything important on your disk before you do this.

Basically, you can't add a disk to a RAID set without formatting it (in any obvious way - read on), and there is also no obvious way to install OS X to a RAID set.

The trick is, there is a non-obvious way to do this: with the command
diskutil enableRAID mirror /dev/disk(whatever)
you can turn on the flags that say a disk is already part of a RAID set. Fill in the (whatever) part to the appropriate device name.

Then, when you add the disk to the RAID set in Disk Utility, it will detect that it's already a part of a mirrored RAID set, and its partner is missing, and will not format it, but make the second disk you add into a copy.

Note I've not done this myself - I have a couple of disks I mean to turn into a RAID set, but I haven't finished moving all my data off the one yet.

[ElDiabloConCaca]

You also need to decide what kind of RAID you want... RAID is a generic term describing multiple-disk setups ("Redundant Array of Independent Devices") that somehow interact. There two most widely-used are mirroring and striping.

If you use two 120GB drives in a mirrored array, then you'll have a maximum RAID capacity of 120GB, since one drive is just a "backup" or mirror of the other. This is good for data integrity, since if one drive fails, you can replace it and rebuild the RAID from the other drive without data loss.

If you use two 120GB drives in a striped array, then you'll have a maximum RAID capacity of 240GB. Striping a disk set writes alternating bits of data, or "stripes" it across both drives. Good for increased speed and decreased latency. Bad thing is, if one drive fails, bye-bye data!

These two are available with Mac OS X's Disk Utility. There are other RAID setups, like striping with parity for error correction/detection and so on. Might wanna do a Google search and research what kind of RAID array you want and what would suit your usage requirements.

If you wanna just "play around" with the whole RAID thing, I would get two identically sized drives (preferrably the same exact drives) and a PCI ATA RAID card. PCI RAID cards do the whole RAID thing in their hardware, whereas Disk Utility does it in software -- and we all know hardware is faster than software.

I would also suggest keeping your boot drive on a separate, non-RAID volume. I doubt you'd see much speed improvement on a desktop workstation like yours simply by putting the OS on a RAID set as opposed to getting a fast non-RAID PCI ATA card and putting your boot drive on that.