RAID Level Classifications

RAID is a term commonly used to classify different types of data-storage patterns used by disk arrays. There are six basic RAID classifications, RAID-0 through RAID-5 and two ways to implement an array.

RAID Level 0

RAID level 0 stripes the data across all of the drives of the array and offers substantial speed enhancement, but provides for no data redundancy. Therefore, a hard disk failure within the array results in loss of data in the logical drive assigned level 0, but only in that logical drive. Note: Although in this case, logical drives assigned level 1 or 5 in the same array do not suffer loss of data, the status of these drives is Critical, and they cannot sustain another hard disk drive failure. They operate at reduced performance. Replace and rebuild the defunct hard disk drive promptly to avoid a multiple hard disk drive failure. When the defunct drive is replaced, all the logical drives assigned RAID levels 5 and 1 are rebuilt onto that hard disk drive; the level-0 logical drive is defined, but the data that was in the defunct level-0 logical drive is lost. Even though the risk of data loss is present, you might want to consider assigning RAID level 0 to one of the logical drives so that you can take advantage of the speed offered with this level.  Hot-spare drive will not be activated after a drive failure in an array that contains a logical drive assigned RAID level 0.  All logical drives in an array containing only one drive are automatically assigned RAID level 0.

RAID Level 1

RAID level 1 provides an enhanced feature for disk mirroring that stripes data and copies of the data across all the drives of the array. The first stripe is the data stripe; the second stripe is the mirror (copy) of the first data stripe, but written on another drive. Because the data is mirrored, the capacity of the logical drive when assigned level 1 is 50% of the physical capacity of the grouping of hard disk drives in the array. RAID level 1 uses two drives; if more than two drives are in the array, your RAID level is automatically switched to Enhanced RAID level 1. If you have three or more drives in the array and want data redundancy, it's a good idea to choose level 5 before level 1.

RAID Levels 2, 3, and 4

Level 2 uses a technique called Bit Interleave Data Striping and level 3 uses a technique called Bit Interleave Data Striping (small stripe) with Parity. Neither of these techniques can handle multiple, simultaneous small requests for data without causing a decrease in performance. RAID level 4 uses a technique similar to level 3 but uses a large stripe.

RAID Level 5

RAID level 5 stripes data and parity across all drives of the array. When a disk array is assigned level 5, the capacity of the logical drive is reduced by one drive (for data parity storage). Level 5 is generally the most desirable choice, because it offers both data protection and increased throughput. Level 5 gives you higher capacity than level 1, but level 1 offers better performance. If after using level 5 you are dissatisfied with the performance and can tolerate lower capacity, you can either redefine the level to level 1, or you can use a logical drive that you have assigned level 1.

RAID Array Implantations

There are two ways to implement a RAID array, the first is a hardware array and the second is a software array. There are limitations on the software implementation of an array. Software arrays or a Dynamic array (Under Windows XP or 2003 Server) cannot be the boot device. This is not true for a hardware array. The reason a Dynamic disk cannot be a boot device is a cause of how the operating system starts. The MBR (Master Boot Record) must be in place just after the POST stage of the boot. Since a software array does not become visible until after the NTLDR has started the Boot Strap Loader cannot find the operating system files. Hardware arrays are preferable and are usually available on either the motherboard or on a separate disk controller card.

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