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A Beginner’s Guide to RAID (Part 1):

The Hard Drive

I think we can safely assume that the majority of our readers have a basic working knowledge of their PC. If not, let me acquaint you with the item with which RAID is concerned. This would be your hard disk drive or HDD. Now your HDD is where all of the software (operating system, spreadsheets, games, etc.) is stored. Without a HDD, you don’t have a PC. Considering the ever-increasing size of HDD’s today, pushing 320GB, this means a lot of data to be stored. I think we can also assume that you would like this data to remain on your HDD for an extended period of time.

As I’m sure you are aware, more and more data on these disks means, progressively, slower and slower read/write times. For example, if I click “save” on this document, this is “writing” to the HDD. If I then close it and open it again, this is “reading” from the HDD. The time it takes to put the information on the disk or open the information from the file is the read and/or write time. As you place more and more data on the disk, space has to be allocated for each item. Unless you regularly defragment your disk and practice proper maintenance, the data can be placed at various locations on the disk. I won’t go into the actual description of HDD, if you are curious you can look here. For our purposes, we’ll skip the HDD 101.

RAID

No, it’s not the can under the sink that Kills Bugs Dead. RAID is an acronym for either: Redundant Array of Inexpensive Disks or Redundant Array of Independent Disks.

The concept of RAID is not a new one, at least in technology terms. It was developed by the computer science department at UC Berkeley back in the 80′s. That’s where the Inexpensive part came from. The term Independent is now in use, but the concept is the same. Fine, you say, but what does all that mean? Well, let’s look at the operative words in the phrase, Redundant and Array. If I say, “That politician is a crook!” I am being redundant, or possibly naïve. I am repeating myself. This is an accurate portrayal of the meaning in this context, for the most part. As you’ll see, you can have a RAID array without the redundancy part. Speaking of an Array, what we mean here is two or more HDD’s grouped together that appear as a single HDD to your CPU. How this is accomplished will be discussed later in Part 1. Depending on where you look you can find a multitude of RAID levels. We’ll be dissecting three of the most common in this section. They are level 0, level 1 and level 0+1, sometimes referred to as 10 or 0/1.

You may often see RAID 5 used also. There are technical differences which are, frankly too complex to discuss in this short article. For our purposes, we shall use 0+1 and consider this “striping with parity”.

At the same time there are two distinct types of RAID. These are software and hardware. Which type of controller you use makes the designation. With hardware RAID control is accomplished by a PCI card, similar to a sound card or through the use of a motherboard with a RAID controller. Software RAID performs this same task as part of the operating system. High-end motherboards today often come equipped with on-board RAID controllers. Your choice of motherboard will often determine which RAID type you choose. The two most common and popular are Promise and High Point. This decision rests with you. I won’t recommend one over the other. Weigh the benefits and costs, and then choose your weapon.

Operation

By now we’ve firmly established the definition of RAID and where in your PC it may come in handy. At this point you are most likely asking, “Yeah, but how does it work?” Thanks for asking; I’d be happy to explain.

As I mentioned above, there are 3 common levels of RAID we plan to focus on. RAID 0, RAID 1 and RAID 0+1. It’s not essential that we decide on a type just yet, so let’s leave that till later. To get things going smoothly, how about we start with the simplest first?

RAID 1

RAID 1, also known as “mirroring”, does just that. This is where it earns its title of redundant. In this setup all the data is written to each disk simultaneously, in effect “mirroring” the data to each disk. I know, a mirror image is backwards, but you get the idea. This is the optimal array if system integrity and data backup is the goal. Your CPU will only see one large logical disk even though you have two or more physical disks.

Remember the read/write speed I mentioned earlier? Well, this is the drawback of using RAID 1. Since the same data must be written on multiple disks, these times might increase. You may be lucky and they won’t, but don’t look for any increases either. However, if you are just looking for a backup you could try a tape drive, zip/jazz drive (for smaller needs) or one of a multitude of software programs. If one of those options will suffice or this doesn’t sound like something you need, why not move on to the next level, RAID 0.

RAID 0

RAID 0, also known as “striping”, is the process of “writing” the data across two or more disks simultaneously. It sounds similar to RAID 1, except that it doesn’t write all of the data to each disk. Striping involves writing portions of the data to each disk. For example, on a two disk RAID 0 array, a section of data will be written to disk 1, another section of data from the same file will be written to disk 2, section 3 will be written to disk 1, section 4 will be written to disk 2 and so on. All of this writing to disks takes place at the same time, just as in RAID 1 except there is no redundancy. There you have the drawback to RAID 0, no data integrity. If one disk fails, the whole array fails because the data is spread between all of the disks. This is a major concern for those of us who don’t use another method of backing up our data. This is a very real option for individuals who have a machine they use for gaming exclusively and want the maximum speed during a marathon fragfest of UT (Unreal Tournament). Gee, I haven’t given you any great reasons to start a RAID array yet, have I? Well, let’s take a gander at another level that might meet your needs, RAID 0+1(0/1, 10)

RAID 0+1

RAID 0+1, also known as RAID 0+1, is the best of both worlds. You get redundancy and the parity (data protection) of RAID 1 and the speed of RAID 0. In order to do this you need a controller that can combine two RAID arrays into a single array or parity group. So essentially you set up your RAID 1 and RAID 0 and they are in turn set up as RAID 0+1. You get high performance combined with data integrity. Well, why didn’t I just start with that one and finish there? Because, you need to know what you are doing before you open that case up and I couldn’t have written this nice guide. But, since you are all fired up about this we will proceed to your next hurdle. What controller do I get?

RAID Types

In order to achieve all the benefits of using RAID, you need a controller. A controller manages the array and transfers of data between the disks. This is the most important part of the array so don’t skimp, find a good one that does what you want. There are two main types, hardware and software. There are advantages and disadvantages to each. Software RAID is just that, software that controls the interaction of the drives and transfer of data. It can generally handle all of the operations needed for running a good RAID setup and is generally easier to set up. Of course, being software based all the functions are handled by your CPU, it can severely tax its power and adversely affect its other activities, and it is also usually not as flexible. Hardware RAID takes this control away from the processor and relieves it of this burden. It can either be incorporated into the motherboard or come in the form of an add-on PCI card. The price can range from $15-$20 more on the motherboard price to $100 or more for the card. All the functions handled by the CPU in the software RAID are now performed by the PCI card or motherboard controller, thus allowing the CPU to work on more important items. Like alt.binaries.whatever. It also offers more robust fault-tolerant features and increased performance. There is the question of disabling if you already own an OS with software RAID and want to switch to hardware. Again, get one that does what you need at an acceptable speed and fits in your budget. We won’t even get into SCSI.

Conclusion

If you haven’t already decided on RAID, do so. It’s a small investment that can make a world of difference. I covered a lot of ground and moved pretty quickly today. If you have any further questions, google, there are a million other amateur Ph.D’s out there and I’m sure you can find one that helps. Thus concludes Part 1 of the Beginner’s Guide to RAID. Be sure to tune in for the next exciting chapter, Part 2: Installation and Operation of RAID, also known as “This program has performed an illegal operation and will be shut down”.

About Jeffery Sexton