A couple of the recent Tech Tips from www.geeks.com have made mention of RAID, but the degree of detail needed in those ideas didn’t shed much light on what RAID actually is. The quantity of e-mail responses and comments in the Viewers Digress area was convincing enough that an introduction to the basics of RAID would be an suitable Tech Suggestion, so here it is.

Introduction

The phrase RAID appears like it may explain some thing Marines conduct in Fallujah, or a can of what all roaches concern, but it is merely an acronym that stands for Redundant Array of Independent (or Inexpensive) Disks. Depending on who you speak to, the letter &quotI&quot can stand for either impartial or affordable, but in my viewpoint impartial is much more suitable, and much less subjective.

RAID generally permits data to be written to multiple hard disk drives so that a failure of any 1 drive in the array does not outcome in the loss of any data, as nicely as growing the system’s fault tolerance. I say RAID generally does this, as there are a number of RAID configurations that provide different approaches to redundancy, but some RAID configurations are not redundant at all. Fault tolerance refers to a system’s ability to continue running when introduced with a hardware (or software program) failure, as should be skilled when a hard drive fails in 1 of the redundant configurations of RAID.

The Hardware

The basic hardware needed to operate RAID includes a set of matched hard drives and a RAID controller.

RAID can be operate on any type of hard drive, including SCSI, SATA, and ATA. The quantity of hard drives needed is dependent on the specific RAID configuration chosen, as described later. I mention the require for matched hard drives, and although this is not absolutely essential, it is recommended. Most arrays will only be able to use the capacity of the smallest drive, so if a 250GB Hitachi drive is additional to a RAID configuration with an 80GB Hitachi drive, that extra 170GB would probably go to waste (the only time that this does not use is in a RAID configuration called JBOD (Just a Bunch Of Disks) which truly &quotisn’t a RAID configuration&quot but just a convenient thing that a RAID controller can do ? see &quotBasic RAID Configurations&quot below for much more information). In addition to matching capacities, it is extremely recommended that drives match in terms of pace and transfer rate as the efficiency of the array would be limited by the weakest drive utilized. One much more region that should be regarded as while matching is the type of hard drive. RAID controllers are generally for either SCSI, SATA, or ATA exclusively, although some systems allow RAID arrays to be operated across controllers of different formats.

The RAID controller is exactly where the data cables from the hard drives are connected, and conducts all of the processing of the data, like the common drive connections found on a motherboard. RAID controllers are obtainable as add on cards, such as this Silicon Picture PCI ATA RAID controller, or integrated into motherboards, such as the SATA RAID controller found on the Asus K8V SE Deluxe (http://www.geeks.com/particulars.asp?invtid=K8VSE-DELUXE). Motherboards that include RAID controllers can be operated without the use of RAID, but the integration is a good characteristic to have if RAID is a consideration. Even for systems without onboard RAID, the relatively reduced price of add on cards can make this part of the upgrade relatively pain totally free.

Another piece of hardware that is not needed, but might show useful in a RAID array is a scorching swappable drive bay. It permits a failed hard drive to be removed from a live method by merely unlocking the bay and sliding the drive cage out of the situation. A new drive can then be slid in, locked into place, and the method will not skip a beat. This is typically seen on SCSI RAID arrays, but some IDE RAIDS cards will also allow this (such as this item produced by Promise Technologies: http://www.guarantee.com/item/item_detail_eng.asp?productId=92&#038familyId=7).

The Software

RAID can be operate on any contemporary running method provided that the suitable drivers are obtainable from the RAID controller’s manufacturer. A pc with the running method and all of the software program currently put in on 1 drive can be easily be cloned to an additional single drive by utilizing software program like Norton Ghost. But it is not as easy when going to RAID, as a consumer who desires to have their existing method with a single bootable hard drive upgraded to RAID must start from the beginning. This implies that the running method and all software program requirements to be re-put in from scratch, and all crucial data must be backed up to be restored on the new RAID array.

If a RAID array is desired in a method for use as storage, but not as the location for the running method, issues get much easier. The existing hard drive can stay intact, and the essential configuration can be made to add the RAID array without beginning from scratch.

Basic RAID Configurations

There are about a dozen different types of RAID that I know of, and I will explain 5 of the much more common configurations, and generally offered on RAID controller cards.

RAID is 1 of the configurations that does not provide redundancy, generating it arguably not a accurate RAID array. Utilizing at least two disks, RAID writes data to the two drives in an alternating fashion, referred to as striping. If you had eight chunks of data, for instance, chunk 1, 3, five, and 7 would be written to the initial drive, and chunk two, four, 6, and eight would be written to the 2nd drive, but all in sequential purchase. This process of splitting the data across drives permits for a theoretical efficiency boost of up to double the pace of a single hard drive, but actual world results will generally not be nearly that good. Because all data is not written to every disk, the failure of any 1 drive in the array generally results in a complete loss of data. RAID is good for individuals who require to access big files quickly, or just demand higher efficiency across the board (i.e. gaming systems). The capacity of a RAID array is equal to the sum of the individual drives. So, if two 160GB Seagate drives were in a RAID array, the total capacity would be 320GB.

RAID 1 is 1 of the most basic arrays that provides redundancy. Utilizing at least two hard drives, all data is written to both drives in a technique referred to as mirroring. Each drive’s contents are identical to every other, so if 1 drive fails, the method could continue running on the remaining good drive, generating it an perfect option for those who value their data. There is no efficiency increase as in RAID , and in fact there might be a slight reduce in comparison to a single drive method as the data is processed and written to both drives. The capacity of a RAID 1 array is equal to half the capacity of the sum of individual drives. Utilizing those exact same two 160GB Seagate drives from over in RAID 1 would outcome in a total capacity of 160GB.

RAID +1, as the title might suggest, is a mixture of RAID and RAID 1. You have the best of both worlds, the efficiency boost of RAID and the redundancy of RAID 1. A minimum of 4 drives is needed to put into action RAID +1, exactly where all data is written in both a mirrored and striped fashion to the 4 drives. Utilizing the eight chunks of data from the instance over, the write pattern would be some thing like this? Chunks 1, 3, five, and 7 would be written to drives 1 and three, and chunks two, four, 6, and eight would be written to drives two and 4, again in a sequential method. If 1 drive should fail, the method and data are still intact. The capacity of a RAID +1 array is equal to half the total capacity of the individual drives. So, utilizing 4 of the 160 GB Seagate drives results in a total capacity of 320GB when configured in RAID +1.

RAID five might be the most potent RAID configuration for the common consumer, with three (or 5) disks needed. Information is striped across all drives in the array, and in addition, parity information is striped as nicely. This parity information is basically a check on the data being written, so even though all data is not being written to all the drives in the array, the parity information can be utilized to reconstruct a misplaced drive in situation of failure. Maybe a bit difficult to explain, so let’s go back again to the instance of the eight chunks of data now being written to 3 drives in a RAID five array. Chunks 1 and two would be written to drive 1 and two respectively, with a corresponding parity chunk being written to drive three. Chunks three and 4 would then be written to drives 1 and three respectively, with the corresponding parity chunk being written to drive two. Chunks 5 and 6 would be written to drives two and three, with the corresponding parity chunk being written to drive 1. Chunks seven and 8 take us back again to the beginning with the data being written to drives 1 and two, and the parity chunk being written to drive three. It may not sound like it, but because of to the parity information being written to the drive not containing that particular bits of information, there is complete redundancy. The capacity of a RAID five array is equal to the sum of the capacities of all the drives utilized, minus 1 drive. So, utilizing three of the 160GB Seagate drives, the total capacity is 320GB when configured in RAID five.

JBOD is an additional non-redundant configuration, which does not truly provide a accurate RAID array. JBOD stands for Just a Bunch Of Disks (or Drives), and that is basically all that it is. RAID controllers that assistance JBOD allow users to ignore the RAID functions obtainable and merely attach drives as they would to a regular drive controller. No redundancy, no efficiency boost, just additional connections for adding much more drives to a method. A smart thing that JBOD does is that it can deal with the odd sized drives as if they are a single volume (therefore a 10GB drive and a 30GB would be seen as a single 40GB drive), so it is good to use if you have a bunch of odd sized drives sitting around ? but otherwise it is better to go with a RAID , 1 or +1 configuration to get the efficiency boost, redundancy or both.

Final Words

Applying RAID might sound daunting to those unfamiliar with the concept, but with some of the much more basic configurations it is not much much more involved than setting up a pc to use a regular drive controller. But, the advantages of RAID over a single drive method much outweigh the extra consideration needed during installation. Dropping data as soon as because of to hard drive failure might be all that is needed to persuade anyone that RAID is right for them, but why wait until that happens.