2. RAID: The Details

2.1. RAID Type: Concatenation

Concatenations are also know as "Simple" RAIDs. A Concatenation is a collection of disks that are "welded" together. Data in a concatenation is layed across the disks in a linear fashion from on disk to the next. So if we've got 3 9G (gig) disks that are made into a Simple RAID, we'll end up with a single 27G virtual disk (volume). When you write data to the disk you'll write to the first disk, and you'll keep writing your data to the first disk until it's full, then you'll start writing to the second disk, and so on. All this is done by the Volume Manager, which is "keeper of the RAID". Concatenation is the cornerstone of RAID.

Now, do you see the problem with this type of RAID? Because we're writing data linearly across the disks, if we only have 7G of data on our RAID we're only using the first disk! The 2 other disks are just sitting there bored and useless. This sucks. We got the big disk we wanted, but it's not any better than a normal disk drive you can buy off the shelves in terms of performance. There has got to be a better way..........

2.2. RAID Type: Striping (RAID-0)

Striping is similar to Concatenation because it will turn a bunch of little disks into a big single virtual disk (volume), but the difference here is that when we write data we write it across ALL the disks. So, when we need to read or write data we're moving really really fast, in fact faster than any one disk could move. There are 2 things to know about RAID-0, they are: stripe width, and columns. They sound scary, but they're totally sweet, let me show you. So, if we're going to read and write across multiple disks in our RAID we need an organized way to go about it. First, we'll have to agree on how much data should be written to a disk before moving to the next; we call that our "stripe width". Then we'll need far kooler term for each disk, a term that allows us to visualize our new RAID better..... "column" sounds kool! Alright, so each disk is a "column" and the amount of data we put on each "column" before moving to the next is our "stripe width".

Let's solidify this. If we're building a RAID-0 with 4 columns, and a stripe width of 128k, what do I have? It might look something like this:

Look good? So, when we start writing to our new RAID, we'll write the first 128k to the first column, then the next 128k to the second column, then the next 128k to the third column, then the next 128k to the fourth column, THEN the next 128k to the first column, and keep going till all the data is written. See? If we were writing a 1M file we'd wrap that one file around all 4 disks almost 3 times! Can you see now where our speed up comes from? SCSI drives can write data at about (depending on what type of drive and what type of SCSI) 20M/s. On our Striped RAID we'd be writing at 80M/s! Kool huh!?

But, now we've got ANOTHER problem. In a Simple RAID if we had, say, 3 9G disks, we'd have 27G of data. Now, if I only wrote 9G of data to that RAID and the third disk died, so what, there is no data on it. (See where I'm going with this?) We'd only be using one of our three disks in a simple. BUT, in a Striped RAID, we could write only 10M of data to the RAID, but if even ONE disk failed, the whole thing would be trash because we wrote it on ALL of the disks. So, how do we solve this one?

2.3. RAID Type: Mirroring (RAID-1)

Mirroring isn't actually a "RAID" like the other forms, but it's a critical component to RAID, so it was honored by being given it's own number. The concept is to create a separate RAID (Simple or RAID0) that is used to duplicate an existing RAID. So, it's literally a mirror image of your RAID. This is done so that if a disk crashes in your RAID the mirror will take over. If one RAID crashes, then the other RAID takes its place. Simple, right?

There's not much to it. However, there is a new problem! This is expensive... really expensive. Let's say you wanted a 27G RAID. So you bought 3 9G drives. In order to mirror it you'll need to buy 3 more 9G drives. If you ever get depressed you'll start thinking: "You know, I just shelled out $400 for 3 more drives, and I don't even get more usable space!". Well, in this industry we all get depressed a lot so, they thought of another kool idea for a RAID......

2.4. RAID Type: Stripping plus Mirroring (RAID-0+1)

When we talk about mirroring (RAID-1) we're not explicitly specifying whether we're mirroring a Simple RAID or a Striped (RAID-0) RAID. RAID-0+1 is a term used to explicitly say that we're mirroring a Striped RAID. The only thing you need to know about it is this...

A mirror is nothing more that another RAID identical to the RAID we're trying to protect. So when we build a mirror we'll need the mirror to be the same type of RAID as the original RAID. If the RAID we want to mirror is a Simple RAID, our mirror then will be a Simple RAID. If we want to mirror a Striped RAID, then we'll want another Striped RAID to mirror the first. Right? So, if you say to me, we're building a RAID-0+1, I know that we're going to mirror a Striped RAID, and the mirror itself is going to be striped as well.

You'll see this term used more often than "RAID-1" simply because a mirror, in and of itself, isn't useful. Again, it's not really a "RAID" in the sense that we mean to use the word.

2.5. RAID Type: RAID-5 (Striping with Parity)

RAID-5 is the ideal solution for maximizing disk space and disk redundancy. It's like Striping (RAID-0) in the fact that we have columns and stripe widths, but when we write data two interesting things happen: the data is written to multiple disks at the same time, and parity is written with the data.

Okey, let's break it down a bit. Let's say we build a RAID-5 out of 4 9G drives. So we'll have 4 columns, and lets say our stripe width is 128k again. The first 128k is written on disks one, two AND three. At the same time it's written a little magic number is written on each disk with the data. That magic number is called the parity. Then, the second 128k of data is written to (watch carefully) disks two, three and four. Again, a parity number is written with that data. The third 128k of data is written to disks three, four and one. (See, we wrapped around). And data keeps being written like that.

Here's the beauty of it. Each piece of our data is on three different disks in the RAID at the same time! Let's look back at our 4 disk raid. We're working normally, writing along, and then SNAP! Disk 3 fails! Are we worried? Not particularly. Because our data is being written to 3 disks per write instead of just one, the RAID is smart enough to just get the data off the other 2 disks it wrote to! Then, once we replace the bad disk with a new one, the RAID "floods" all the data back onto the disk from the data on the other 2 adjacent disks! But, you ask, how does the RAID know it's giving you the correct data? Because of our parity. When the data was written to disk(s) that parity was written with it. We (actually the computer does this automatically) just look at the data on disks 2 and 4, then compare (XOR) the parity written with the data and if the parity checks out, we know the data is good. Kool huh?

Now, as you might expect, this isn't perfect either. Why? Okey, number 1, remember that parity that saves our butt and makes sure our data is good? Well, as you might expect the systems CPU has to calculate that, which isn't hard but we're still wasting CPU cycles for the RAID, which means if the system is really loaded we may need to (eek!) wait. This is the "performance hit" you'll hear people talk about. Also, we're writing to 3 disks at a time for the SAME data, which means we're using up I/O bandwidth and not getting a real boost out of it.

2.6. RAID Comparison: RAID0+1 vs RAID5

There are battles fought in the storage arena, much like the old UNIX vs NT battles. We tend to fight over RAID0+1 vs RAID5. The fact is that RAID5 is advantageous because we use less disks in the endeavor to provide large amounts of disk space, while still having protection. All that means is that RAID5 is inexpensive compared to RAID0+1 where we'll need double the amount of disk we expect to use, because we'll only need a third more disks rather than twice as many. But, then RAID5 is also slower than RAID0+1 because of that damned parity. If you really want speed, you'll need to bite the bullet and use RAID0+1 because even though you need more disks, you don't need to calculate anything, you just dump the data to the disks. In my estimates (this isn't scientific, just what I've noticed by experience) RAID0+1 is about 20%-30% faster than RAID5.

Now, in the real world, you rarely have much choice, and the way to go is clear. If you're given 10 9G disks and are told to create a 60G RAID, and you can't buy more disks, you'll need to either go RAID5, or be unprotected. However, if you've got thoughs same disks and they only want 36G RAID you can go RAID0+1, with the only drawback that they won't have much room to grow. It's all up to you as an admin, but always take growth into account. Look at what you've got, downtime availability to grow when needed, budget, performance needs, etc, etc, etc. Welcome to the world of capacity planning!