Basic RAID Levels defined

The various RAID types used in the storage world are defined by Level numbers. At the basic level, we have RAID Level 0 through 6. We also have various composite RAID types comprised of multiple RAID levels. Note that people often drop the word “Level” when referring to RAID types and this has become an accepted practice. Also note that even though same-sized hard drives are not technically required, RAID normally uses hard drives of similar size. Any implementation that uses different sized hard drives will result in wasted capacity.

RAID Level 0:
RAID Level 0 is the cluster-level implementation of data striping and it is the only RAID type that doesn’t care about fault tolerance. Clusters can vary in size and are user-definable but they are typically blocks of 64 thousand bytes. The clusters are evenly distributed across multiple hard drives. It’s used by people who don’t care about data integrity if a single drive fails. This RAID type is sometimes used by video editing professionals who are only using the drive as a temporary work space. It’s also used by some PC enthusiasts who want maximum throughput and capacity.

RAID Level 1:
RAID Level 1 is the pure implementation of data mirroring. In a nutshell RAID Level 1 gives you fault tolerance but it cuts your usable capacity in half and it offers excellent throughput and I/O performance. This RAID level is often used in servers for the system partition for enhanced reliability but PC enthusiasts can also get a nice performance boost from RAID Level 1. Using multiple independent RAID Level 1 volumes can offer the best performance for database storage.

RAID Level 2:
RAID Level 2 is a bit-level implementation of data striping with parity. The bits are evenly distributed across multiple hard drives and one of the drives in the RAID is designated to store parity. Out of an array with “N” number of drives, the total capacity is equal to the sum of “N-1″ hard drives. For example, an array with 6 equal sized hard drives will have the combined capacity of 5 hard drives. It’s interesting to note that this RAID level is almost forgotten and is very rarely used.

RAID Level 3:
RAID Level 3 is a byte-level implementation of data striping with parity. The bytes are evenly distributed across multiple hard drives and one of the drives in the RAID is designated to store parity. Out of an array with “N” number of drives, the total capacity is equal to the sum of “N-1″ hard drives. For example, an array with 4 equal sized hard drives will have the combined capacity of 3 hard drives. This RAID level is not so commonly used and is rarely supported.

RAID Level 4:
RAID Level 4 is a cluster-level implementation of data striping with parity. Clusters can vary in size and are user-definable but they are typically blocks of 64 thousand bytes. The clusters are evenly distributed across multiple hard drives and one of the drives in the RAID is designated to store parity. Out of an array with “N” number of drives, the total capacity is equal to the sum of “N-1″ hard drives. For example, an array with 8 equal sized hard drives will have the combined capacity of 7 hard drives. This RAID level is not so commonly used and is rarely supported.

RAID Level 5:
RAID Level 5 is a cluster-level implementation of data striping with DISTRIBUTED parity for enhanced performance. Clusters can vary in size and are user-definable but they are typically blocks of 64 thousand bytes. The clusters and parity are evenly distributed across multiple hard drives and this provides better performance than using a single drive for parity. Out of an array with “N” number of drives, the total capacity is equal to the sum of “N-1″ hard drives. For example, an array with 7 equal sized hard drives will have the combined capacity of 6 hard drives. This is the most common implementation of data striping with parity.

RAID Level 6:
RAID Level 6 is a cluster-level implementation of data striping with DUAL distributed parity for enhanced fault tolerance. It’s very similar to RAID Level 5 but it uses the equivalent capacity of two hard drives to store parity. RAID Level 6 is used in high-end RAID systems but it’s slowly becoming more common as technology becomes more commoditized. Dual parity allows ANY two hard drives in the array to fail without data loss which is unique in all the basic RAID types. If a drive fails in a RAID Level 5 array, you better hope there is a hot spare that will quickly restore the array to a healthy state in a few hours and you don’t get a second failure during that recovery time. RAID Level 6 allows that second drive failure during recovery and is considered the ultimate RAID Level for fault tolerance. Out of an array with “N” number of drives, the total capacity is equal to the sum of “N-2″ hard drives. For example, an array with 8 equal sized hard drives will have the combined capacity of 6 hard drives.

RAID Level 10 (composite of 1 and 0):
RAID Level 10 (sometimes called 1+0) is probably the most common composite RAID type used on the market both in the server and home/enthusiast market. For example, there are plenty of cheap consumer-grade RAID controllers that might support RAID Level 0, 1, and 10 that don’t support Level 5. The most common and recommended implementation of mirroring and striping is that mirroring is done before striping. This provides better fault tolerance because it can statistically survive more often with multiple drive failures and performance isn’t degraded as much when a single drive has failed in the array. RAID Level 0+1 which does striping before mirroring is considered an inferior form of RAID and is not recommended. RAID Level 10 is very commonly used in database applications because it provides good I/O performance when the application can’t distribute its own data across multiple storage volumes. But when the application knows how to evenly distribute data across multiple volumes, independent pairs of RAID Level 1 provides superior performance.

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