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
