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What is RAID?

RAID is an acronym for "Redundant Array of Independent Drives," or "Redundant Array of Inexpensive Drives." The main idea behind RAID is the ability to take multiple drives and have them virtualised as a single drive. There are many different RAID structures, all of obtain one of two primary purposes: more storage space or more data redundancy (as in, protection against data loss in the even of hard drive failure). I'm not going into the details of all the RAID levels as this post is more geared towards the lower level workings of RAID-5, but if you wish to learn about all the RAID levels, see this Wikipedia Article On RAID.

What is RAID 5?

RAID 5 provides a very redundant fault tolerance with performance advantages that allows data to be safeguarded while only sacrificing the equivalent of one drive's space. Level 5 requires at least 3 hard drives of the same size; The total storage space available with a RAID 5 is equal to { (number of drives - 1) * size of smallest drive }. So if you use three 120gb hard drives, you will have 240gb of actual usable space. If you used five 120gb hard drives, you would have 480gb of usable space. The more drives you use, the more efficient your storage space is.

Data Redundancy

Your data can survive a complete failure of one hard drive, but if two drives fail at the same time, ALL data is lost. It is very important to have an extra drive on hand so that if a drive fails, you can replace it immediately for data rebuild. The RAID array can actually still be used with one drive completely missing or not working, but performance is degraded as the data must be rebuilt on the fly; however If you do not have an extra drive to plug in right away when one fails, it would be wise to keep the computer and all drives powered off until you can replace the failed drive. You may think "oh it will only be a couple days before the new drive arrives", but ask yourself this: Is having access to the data on these drives for only a couple days worth taking the risk of losing it all forever if another drive happens to fail? Probably not.

Figure 1. Representation of RAID-5 data structure.

Striping & Parity

Data is "striped" across the hard drives, with a dedicated parity block for each stripe. A, B, C, and D represent data "stripes." Each stripe segment per drive can vary in size; I believe anywhere from 4kb to 256kb per stripe is normal and can be set during setup to adjust performance. The blocks with a subscript P are the parity blocks which are a representation of the sum of all other blocks in that stripe (explained in more detail below). The parity is responsible for the data fault tolerance and is also the reason why you lose the amount of space equivalent to one drive. Using figure 1, lets say that the second drive gets corrupt and dies. A new hard drive would be put in its place, and the RAID controller would rebuild the data automatically. The data in segments A1, A3 would be compared to the A_P parity block, which would allow the data for A2 to be rebuilt -- as goes for all other stripes that must be recovered. Parity blocks are determined by using a logical comparison called XOR (Exclusive OR) on binary blocks of data which will be explained down further.

Performance

RAID 5 offers accelerated read performance because the data stream is accessed from multiple drives at the same time. Referring to figure 1, lets say that stripe A was a single file. Normally on a single drive, when you open that file, the whole thing would have to be streamed from the one hard drive, thus the one hard drive's max read speed is going to become a bottleneck. BUT, with a RAID 5, that one file can be accessed in 1/3 the time because it will be read from all 3 drives at once; block 1 has the first 1/3 of the file, block 2 has the second 1/3 section of the file, and the block 3 has the last part of the file. This, in the perfect situation, causes your read speed to be tripled -- with even more performance potential with arrays containing more hard drives!

The downfall to this is that there is an additional overhead when writing to the disc. This overhead is caused from parity calculation. Every single bit written to the drives must be compared and processed to create a parity block. If your intended use involves a lot of data writing (such as video recording, high traffic server, etc) raid 5 would not be the most ideal choice.

XOR Comparison

As I'm sure you already know, data is stored and processed in binary, which is of course 0's and 1's. There are methods of comparing binary bits called operators. The one that does the magic of parity creation is called XOR, or Exclusive OR. If you have experience in lower level programming or electronics, you probably already know what an XOR is.

Figure 2. XOR Inputs/Outputs

Basically, an XOR will take two binary bits, compare them, and output a result of 0 or 1. It will return a 1 ONLY IF the two inputs are different. If both bits are 0, the output is 0. If both bits are 1, the output is 0. If one bit is 0, and the other bit is 1, the output is 1.

Figure 3. Yellow cells represent parity blocks.

Building Parity

For easier understanding/explaining, we are only going to be working with 4-bit blocks; Actual data blocks can range from 4kb (32,768 bits) up to 256kb (2,097,152 bits), but the method is exactly the same regardless of how many consecutive bits you work with. In figure 3, the yellow blocks represent the parities for each stripe. As you probably notice right away, how the parities are distributed between all drives; This provides a slight increase in performance, and is was separates RAID 4 from RAID 5 (RAID 4 keeps all parities on a single drive).

Lets examine the first stripe of figure 3. To compute the parity, we must run the XOR comparison on each block of data in that stripe. You XOR the first two blocks, then take the result, and XOR it against the third block. (and continue this for all discs in the array, except the block where the parity will be stored, of course.)

(Drive 1) XOR (Drive 2) = (0100) XOR (0101) = (0001) (Result) XOR (Drive 3) = (0001) XOR (0010) = (0011)

Let me break that down a little more in case you couldn't follow. Refer to figure 2 if you have trouble remembering the inputs/outputs for XOR

First we need to compare the first two drives' blocks which are 0100 and 0101. The very first bit comparison is 0 and 0 (the first bits from both blocks) which results 0, the first bit of our temporary parity. The second bits are 1 and 1, which results 0. So far, our temporary parity is 00. Now the third bit comparison is 0 and 0, which yet again, returns 0. We are now at 000. The fourth bit comparison is 0 and 1, which results 1. So the result of (Drive 1)XOR(Drive 2) is 0001. We now must take this block, and compare it to drive 3 which is 0010. The XOR of 0001 and 0010 equals 0011, which is the parity for stripe 1!

Recovering Data

The very cool thing about XOR comparisons, and what makes RAID 5 possible, is that if one value comes up missing, you can always find the missing value by running an XOR on all the available values! Referring back to diagram 3, lets say that drive 1 failed beyond fixing. The user will be prompted by the raid controller and alerted that a drive has failed, and must be replaced. As soon as a new drive is put in, the controller will automatically start rebuilding the lost data. Here is how we rebuild drive 1, stripe 1

(Drive 2) XOR (Drive 3) = (0101) XOR (0010) = (0111) (Result) XOR (Drive 4) = (0111) XOR (0011) = (0100)

As you can see, the final result is 0100, now refer back to figure 3 at drive 1, stripe 1.... sure enough, its 0100! Amazingly, right? Just for fun, lets rebuild stripe 2 as well assuming drive 1 died.

(Drive 2) XOR (Drive 3) = (0000) XOR (0110) = (0110) (Result) XOR (Drive 4) = (0110) XOR (0100) = (0010)

The missing block was calculated as 0010. Take a look at figure 3 to verify what drive 1, stripe 2 was before the failure and see if it matches the computed value... of course it does!

Well I hope you have enjoyed this post. It took me a great deal of searching to find this data when my own curiosity got to me, and I couldn't find any articles that explained all of this in one, so I decided to write this article and hope someone finds it helpful!

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