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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Redundant Arrays of Inexpensive Disks (RAID) | RAID Levels | Single RAID Levels ]

RAID Level 4

Common Name(s): RAID 4 (sometimes called RAID 3 by the confused).

Technique(s) Used: Block-level striping with dedicated parity.

Description: RAID 4 improves performance by striping data across many disks in blocks, and provides fault tolerance through a dedicated parity disk. This makes it in some ways the "middle sibling" in a family of close relatives, RAID levels 3, 4 and 5. It is like RAID 3 except that it uses blocks instead of bytes for striping, and like RAID 5 except that it uses dedicated parity instead of distributed parity. Going from byte to block striping improves random access performance compared to RAID 3, but the dedicated parity disk remains a bottleneck, especially for random write performance. Fault tolerance, format efficiency and many other attributes are the same as for RAID 3 and RAID 5.

This illustration shows how files of different sizes are distributed between
the drives on a four-disk RAID 4 array using a 16 kiB stripe size. As with the
RAID 0 illustration, the red file is 4 kiB in size; the blue is 20 kiB; the green
is 100 kiB; and the magenta is 500 kiB, with each vertical pixel representing
1 kiB of space. Notice that as with RAID 3, the files are evenly spread between
three drives, with the fourth containing parity information (shown in gray).
You may want to contrast this illustration to the one for RAID 3 (which is very
similar except that the blocks are so tiny you can't see them) and the one
for RAID 5 (which distributes the parity blocks across all four drives.)

Controller Requirements: Generally requires a medium-to-high-end hardware RAID card.

Hard Disk Requirements: Minimum of three standard hard disks; maximum set by controller. Should be of identical size and type.

Array Capacity: (Size of Smallest Drive) * (Number of Drives - 1).

Storage Efficiency: If all drives are the same size, ( (Number of Drives - 1) / Number of Drives).

Fault Tolerance: Good. Can tolerate loss of one drive.

Availability: Very good. Hot sparing and automatic rebuild are usually supported..

Degradation and Rebuilding: Moderate degrading if a drive fails; potentially lengthy rebuilds.

Random Read Performance: Very good.

Random Write Performance: Poor to fair, due to parity calculation overhead and the bottleneck of the dedicated parity drive.

Sequential Read Performance: Good to very good.

Sequential Write Performance: Fair to good.

Cost: Moderate. A hardware controller is usually required, as well as at least three drives.

Special Considerations: Performance will depend to some extent upon the stripe size chosen.

Recommended Uses: Jack of all trades and master of none, RAID 4 is not as commonly used as RAID 3 and RAID 5, because it is in some ways a "compromise" between them that doesn't have a target market as well defined as either of those two levels. It is sometimes used by applications commonly seen using RAID 3 or RAID 5, running the gamut from databases and enterprise planning systems to serving large multimedia files.

Next: RAID Level 5


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