XCPU’s Guide To RAID

Overall Score

Now, on to the actual meat and potatoes:

What how many kinds of RAID are there?
Well, there are at least nine types of RAID plus the non-redundant array which is known as RAID 0.

What uses do those 9 kinds have?

  • RAID-0: This technique has striping but no redundancy of data. It offers the best performance but no fault-tolerance.

    Please note, any disk failure destroys the array, which becomes more likely with more disks in the array. The reason a single disk failure destroys the entire array is because when data is written to a RAID 0 drive, the data is broken into "fragments". The number of fragments is dictated by the number of disks in the drive.

    Each of these fragments are written to their respective disks simultaneously on the same sector. This allows the entire chunk of data to be read off the drive in parallel, giving this type of arrangement huge bandwidth. When one sector on one of the disks fails though, the corresponding sector on every other disk is rendered useless because part of the data is now corrupted. RAID 0 does not implement error checking so any error is unrecoverable. More disks in the drive means higher bandwidth, but greater risk of data loss.

RAID Level 0 requires a minimum of 2 drives to implement

  • RAID-1: This type is also known as disk mirroring and consists of at least two drives that duplicate the storage of data. There is no striping. Read performance is improved since either disk can be read at the same time. Write performance is the same as for single disk storage. RAID-1 provides the best performance and the best fault-tolerance in a multi-user system.

For Highest performance, the controller must be able to perform two concurrent separate Reads per mirrored pair or two duplicate Writes per mirrored pair.
RAID Level 1 requires a minimum of 2 drives to implement

  • RAID-2: This type uses striping across disks with some disks storingECC information. It has no advantage over RAID-3.

Each bit of data word is written to a data disk drive (4 in this example: 0 to 3). Each data word has its Hamming Code ECC word recorded on the ECC disks. On Read, the ECC code verifies correct data or corrects single disk errors.

  • RAID-3: This type uses striping and dedicates one drive to storing parity information. The embedded error checking information is used to detect errors. Data recovery is accomplished by calculatingXOR of the information recorded on the other drives. Since an I/O operation addresses all drives at the same time, RAID-3 cannot overlap I/O. For this reason, RAID-3 is best for single-user systems with long record applications.

The data block is subdivided ("striped") and written on the data disks. Stripe parity is generated on Writes, recorded on the parity disk and checked on Reads.
RAID Level 3 requires a minimum of 3 drives to implement

  • RAID-4: This type uses large stripes, which means you can read records from any single drive. This allows you to take advantage of overlapped I/O for read operations. Since all write operations have to update the parity drive, no I/O overlapping is possible. RAID-4 offers no advantage over RAID-5.

Each entire block is written onto a data disk. Parity for same rank blocks is generated on Writes, recorded on the parity disk and checked on Reads.
RAID Level 4 requires a minimum of 3 drives to implement

  • RAID-5: This type includes a rotating parity array, thus addressing the write limitation in RAID-4. Thus, all read and write operations can be overlapped. RAID-5 stores parity information but not redundant data (but parity information can be used to reconstruct data). RAID-5 requires at least three and usually five disks for the array. It’s best for multi-user systems in which performance is not critical or which do few write operations.

Each entire data block is written on a data disk; parity for blocks in the same rank is generated on Writes, recorded in a distributed location and checked on Reads.
RAID Level 5 requires a minimum of 3 drives to implement

  • RAID-6: This type is similar to RAID-5 but includes a second parity scheme that is distributed across different drives and thus offers extremely high fault- and drive-failure tolerance.


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