Redundant Array of Independent Disks

Depending on the bibliography, the "I" on RAID stands for “Inexpensive”.

RAID is a hard disk drive array that can be assembled with the following goals:

• Mirroring: Automatically copies, thru hardware, all data sent to one hard disk drive to another hard drive. If the first hard drive fails, the second disk, which has a backup of all data stored in the first disk, enters in action automatically.
• Data stripping: Increases the storage system performance by accessing two or more hard disk drives in parallel. The files aren’t stored in just one hard disk drive; they are split and each part of the file is written on a different hard drive. Since each hard drive will store just a small part of the file and not the whole file, the storage is done faster.

RAID systems can be classified like this:

• RAID 0: Data stripping. Needs at least two hard disk drives.
• RAID 1: Mirroring. Needs two hard disk drives with the same capacity.
• RAID 0+1: System using, at the same time, data stripping and data mirroring. It needs four hard disk drives. Two will be used for data stripping and the other two will be used to mirror the first two. Take care to not confuse this system with RAID10. On RAID 0+1 if one of the hard drives fails, the system becomes a RAID 0 system. On RAID10, if one of the hard drives fails, the system becomes a RAID 1 system.
• RAID JBOD: RAID0+1 using just two hard disk drives. To make this possible, only half of each hard disk is used to store data, in order to simulate the presence of four hard disk drives.
• RAID 10: Similar to RAID 0+1, its main difference is if one of the hard drives fails, the system becomes a RAID 1 system. On RAID 0+1 if one of the hard drives fails, the system becomes a RAID 0 system.
• RAID2: Similar to RAID0, but with error correction scheme (ECC);
• RAID3: Similar to RAID0, but using an extra hard disk for parity information storage, thus enhancing the system reliability;
• RAID4: Similar to RAID3, but faster by using larger data chunks, i.e. the files are stripped into larger blocks;
• RAID5: Similar to RAID3 and RAID4, but saving the parity information inside the data disks not on an extra disk, so you won't need an extra disk;
• RAID53: Similar to RAID3 but using at least 5 hard disks, to enhance the system performance;
• RAID6: Based on RAID5, it saves an extra parity information on all hard disks of the system, enhancing the system reliability;
• RAID7: Trademark from a company called Storage Computer Corporation, it uses an extra disk to save parity information. Its main advantage is its speed, because it uses disk cache technique. It can be considered a RAID4 with disk cache.

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