can someone explain RAID storage?

This is a great Visual: http://www.acnc.com/raid.html
And this has more detail: http://en.wikipedia.org/wiki/Redundant_array_of_independent_disks

 

This Wikipedia article should explain all you need, but it gets pretty advanced. Here are the basics that you'd probably be interested in.

http://en.wikipedia.org/wiki/RAID

First, most RAID is redundant. RAID was designed to be redundant. The idea, basically, is that you mirror the same data over more than one hard drive, so if one fails you lose no data.

However, there are speed benefits as well. By having twice as many readers over the same data, the hard drive performs faster than a single hard drive. The disadvantage for redundant RAID is that, for example, if you put in two 100GB hard drives, you only get 100GB of storage.

The other kind is RAID 0. It came after the original types of RAID, and is just used to increase performance. It ties two hard drives together (two 100GB hard drives gives you 200GB of storage), and the additional read head makes it that much faster. However, it isn't redundant. If a drive fails, you're SOL.

The two basic RAID levels are 0 and 1. 0 is non-redundant, 1 is redundant. Most entry-level RAID controllers will only support these levels. 2 through 5 are more advanced. They simply differ in exactly how they mirror the data over multiple drives, with varying advantages to each.

EDIT: To make it clear, I over-simplified a lot. This is just intended as a primer for the basics; don't take it as holy writ.

 

Keep it simple! I admin a few RAIDS, one of which is doubly redundant- 2 RAID5s that are mirrors of each other. With this set up, there is virtually no down time or possibility of data loss. I can loose a single drive with no downtime, I can loose a single RAID with little down time. I also don't have the option of loosing any data or having much down time.- But this is work, and I work with lots of large audio files. These RAIDs are not OS X based.

For home, I have once 250 Gb drive with 2 partitions, one bootable clone of my computer, and the other with an incremental backup. I have 1 250Gb drive for clips, and one for AV rendering and as a scratch disk, and finally one 250Gb drive for mass media storage.

RAIDS have their advantages and disadvantages, but in reality, keeping it simple is best. OS X Server offers a lot of easily accessible tools, OS X consumer edition only offers limited RAID support and repair tools, many of which are only accessible through terminal commands.

 

what software do you use to keep a bootable clone of your computer?

 

CCC, Carbon Copy Cloner it has never failed me. Free limitless trial, 5 dollar shareware. No annoying pop ups, but man it's 5 bucks pay the guy.

Other like SuperDuper. ~$27.

If you have a Lacie drive, it likely came with SilverKeeper Lacie's free backup app. Also on their site they offer a program called OneTouch for free to Lacie owners.

There are other cloning applications.

For the mirror of my primary RAID, I just use a "simple" script.

 

If you're using Mac OS X's software RAID solution, does this mean the RAIDset breaks if you reformat the boot volume and reinstall OS X? What happens if you do this and install a newer version of the OS (eg, Tiger -> Leopard)

Will the drives remember that they are part of a RAID array? Indeed, will the newly installed OS X recognise the fact some drives are part of an array?

I'm just scared of setting up a RAID array then at some point in the future reinstalling OS X from scratch and losing all the data on the other RAID disks...

 

Remember that RAID 1 - 5, 1+0 and 0+1 only protects against single hard drive mechanism failure. Two simultaneous drive failures usually means you're hooplated. A striped volume is way more difficult to do data recovery on.

It does not protect against procedural errors (accidentally trashing a file, or overwriting a file or folder with one of the same name) because the Oops moment is instantaneously written to both the master and the mirror (or the disk and the parity disk). It also does not protect against data corruption from crashes, bad RAM, cable or power supply failures, virii and trojans in the wider OS world, etc.

Before you think of a RAID, first think of what your backup strategy is. Then think of your RAID strategy.

 

That is a good point RAID is NOT a replacement for backup. It is just a added measure for systems that can't have much down time, such as servers.
[random>]However servers also normally have other things to help with having less down time such as a second power supply for fall-over protection. They also tend to have a UPS multiple fans so some can die without the need to have the BIOS automatically shut down due to over heating. ECC ram to help prevents errors. And all these items generally are hot swappable. So when thinking of why you want to setup a RAID system think of some of the other things that could happen and also think of what down time means to you. Also how much are you willing to spend to keep things up and running.