Hardware Reference
In-Depth Information
RAID 5
RAID 5 uses three or more physical hard drives. The RAID 5 controller
divides data that is to be written to the array into blocks and calculates
parity blocks for the data. Data blocks and parity blocks are interleaved
on each physical drive, so each of the three or more drives in the array
contains both data blocks and parity blocks. If any one drive in the RAID 5
fails, the data blocks contained on the failed drive can be re-created from
the parity data stored on the surviving drives.
RAID 5 is optimized for the type of disk usage common in an office
environment—many random reads and fewer random writes of relatively
small files. RAID 5 reads are faster than those from a single drive, because
RAID 5 has three or more spindles spinning and delivering data simulta-
neously. RAID 5 writes are typically a bit faster than single-drive writes.
RAID 5 uses hard drive space more efficiently than RAID 1.
In effect, although RAID 5 uses distributed parity, a RAID 5 array can be
thought of as dedicating one of its physical drives to parity data. For ex-
ample, with three 2 TB drives in a RAID 5 array, 4 TB—the capacity of two of
the three drives—is visible to the system. With RAID 5 and four 2 TB drives,
6 TB—the capacity of three of the four drives—is visible to the system. RAID
5 may be implemented with a physical RAID 5 controller or in software by
the operating system. Few motherboards have embedded RAID 5 support.
RAID 3
RAID 3 uses three or more physical hard drives. One drive is dedicated to
storing parity data, with user data distributed among the other drives in
the array. RAID 3 is the least common RAID level used for PC servers, be-
cause its characteristics are not optimal for the disk usage patterns typical
of small office LANs. RAID 3 is optimized for sequential reads of very large
files, so it is used primarily for applications such as streaming video.
Then there is the so-called RAID 0, which isn't really RAID at all because it pro-
vides no redundancy:
RAID 0
RAID 0, also called striping , uses two physical hard drives. Data written to
the array is divided into blocks, which are written alternately to each drive.
For example, if you write a 1 MB file to a RAID 0 that uses 256 KB blocks,
the first 256 KB block may be written to the first drive in the array. The
second 256 KB block is written to the second drive, the third 256 KB block
to the first drive, and the final 256 KB block to the second drive. The file
itself exists only as fragments distributed across both physical drives, so if
either drive fails all the data on the array is lost. That means data stored
on a RAID 0 is more at risk than data stored on a single drive, so in that
sense a RAID 0 can actually be thought of as less redundant than the zero
redundancy of a single drive. RAID 0 is used because it provides the fastest
possible disk performance. Reads and writes are very fast, because they
can use the combined bandwidth of two drives. RAID 0 is a poor choice for
desktops and workstations, which typically do not load the disk subsystem
Search WWH ::




Custom Search