Common Name(s): RAID 3. (Watch out for some companies that say their
products implement RAID 3 when they are really RAID 4.)
Technique(s) Used: Byte-level striping with dedicated parity.
Description: Under RAID 3, data is striped across multiple disks at a
byte level; the exact number of bytes sent in each stripe varies but is typically under
1024. The parity information is sent to a dedicated parity disk, but the failure of any
disk in the array can be tolerated (i.e., the dedicated parity disk doesn't represent a
single point of failure in the array.) The dedicated parity disk does generally
serve as a performance bottleneck, especially for random writes, because it must be
accessed any time anything is sent to the array; this is contrasted to distributed-parity
levels such as RAID 5 which improve write performance by using distributed parity (though
they still suffer from large overheads on writes, as
described here). RAID 3 differs from RAID 4 only in the size of the stripes sent to
the various disks.
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This illustration shows how files of different sizes
are distributed
between the drives on a four-disk, byte-striped RAID 3 array. 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 the files are evenly
spread between three drives, with the fourth containing parity
information (shown in dark gray). Since the blocks are so tiny in
RAID 3, the individual boundaries between stripes can't be seen.
You may want to compare this illustration to the one for RAID 4. |
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 by controllers that implement RAID 3.
Degradation and Rebuilding: Relatively little degrading of performance
if a drive fails. Rebuilds can take many hours.
Random Read Performance: Good, but not great, due to byte-level
striping.
Random Write Performance: Poor, due to byte-level striping, parity
calculation overhead, and the bottleneck of the dedicated parity drive.
Sequential Read Performance: 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: Not as popular as many of the other
commonly-implemented RAID levels. For transactional environments, RAID 5 is usually a
better choice.
Recommended Uses: Applications working with large files that require
high transfer performance with redundancy, especially serving or editing large files:
multimedia, publishing and so on. RAID 3 is often used for the same sorts of applications
that would typically see the use of RAID 0, where the lack of fault tolerance of RAID 0
makes it unacceptable.
Next: RAID Level 4
