Biomedical Engineering Reference
In-Depth Information
Disks (RAID system) that function in part as an integrated archival system. The idea behind a RAID
system is to provide real-time backup of data by increasing the odds that data written to a server will
survive the crash of any given hard drive in the array. RAID was originally introduced in the late
1980s as a means of turning relatively slow and inexpensive hard disks into fast, large-capacity,
more reliable storage systems.
RAID systems derive their speed from reading and writing to multiple disks in parallel. The increased
reliability is achieved through mirroring or replicating data across the array and by using error-
detection and correction schemes. Although there are seven levels of RAID, level 3 is most applicable
to bioinformatics computing. In RAID-3, a disk is dedicated to storing a parity bit—an extra bit used
to determine the accuracy of data transfer—for error detection and correction. If analysis of the
parity bit indicates an error, the faulty disk can be identified and replaced. The data can be
reconstructed by using the remaining disks and the parity disk.
For example, in
Figure 2-10
, disks A-D are dedicated to data and disk P is used to store the parity
bit. In this case, an odd number of "1" bits corresponds to a high ("1") parity bit. When data are
written in parallel to the data disks, the corresponding parity bit is stored on the parity disk.
Immediately after the data are written to the data disks, the data are read and the parity bits are
compared. The discrepancy noted in
Figure 2-10
is typical of a case when there is an error on one
disk. The error on disk "C" can be repaired, or if groups of errors are suddenly becoming apparent
indicating imminent disk failure, then the entire disk can be replaced.
Figure 2-10. RAID-3. Data disks are read and written to in parallel,
providing speed, while a dedicated parity disk provides increased reliability
through error detection and correction. In this example, an error in disk C is
detected by a different parity bit (P), indicating that the data read from
disks A-D don't agree with what was written to the disks. Although the
parity bit is usually based on a comparison of bytes on the data disks, bits
(0 or 1) are used here for clarity.
Another approach is to create archives on separate media on a regular basis and transport the media
offsite to a safe location that would survive natural or man-made disaster at the main computing
