What is RAID?

What is RAID (Redundant Array of Independent Disks)?


RAID creates a single usable data disk, where several physical disks are combined into an array for better speed and/or fault tolerance. There are three key concepts in RAID: mirroring, the copying of data to more than one disk; striping, the splitting of data across more than one disk; and error correction, where redundant data is stored to allow problems to be detected and possibly fixed (known as fault tolerance). Although there are many different levels of RAID, Softlayer chooses to support the most common raid types: 0, 1, 5, and 10, the different RAID levels use one or more of these techniques, depending on the system requirements. The main aims of using RAID are to improve reliability.utilizing either 3Ware 9550SX Raid SATA or an Adaptec SA-SCSI RAID controller for all RAID solutions deployed.


RAID 0 (Striped set without parity/Non-Redundant Array) Implements data striping where file blocks are written across multiple drives in fragments and requires a minimum of 2 disks. The advantage of a RAID 0 is that the read/write speed is dramatically increased. The more disks in the array the greater the bandwidth. The disadvantage to a RAID0 is that there is no fault tolerance; if a single drive fails it will destroy the array. Also a RAID 0 does not implement error checking so any error is also unrecoverable. A common solution to this is to have a drive outside of the array used as back-up storage in case of a hardware failure.

RAID 1 (Mirrored set without parity) Implements data mirroring. Data is duplicated on two or four drives through a hardware raid controller and provides some fault tolerance. The array is recoverable as long as at least 1 drive has not failed. It provides faster read performance than a single drive and provides drive redundancy in case of drive failure. There is also a very slight reduction to write speed.

RAID 5 (Striped set with dual distributed parity) Implements data striping at a block level, and distributes parity among the drives. The parity information allows recovery from the failure of any single drive because any following reads can be calculated from the distributed parity. Another advantage of a Raid 5 allows for increased read/write speeds while allowing the most efficient use of disk space. RAID 5 requires a minimum of 3 disks.

RAID 10 (RAID 1 + 0) Creates multiple mirrors, where data is organized as stripes across multiple disks and then the striped disk sets are mirrored. RAID 10 offers the same fault tolerance as RAID 1 with increased read/write speeds over a single Raid 1 volume or single drive. RAID Level 10 requires 4 drives to implement.

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