Page 181 - DCAP403_Operating System

Operating System

Notes is a way of increasing the disk transfer rate up to a factor of N, by splitting files across N different

disks. Instead of saving all the data from a given file on one disk, it is split across many. Since the
N heads can now search independently, the speed of transfer is, in principle, increased manifold.
Logical disk data/blocks can be written on two or more separate physical disks which can
further transfer their sub-blocks in parallel. The total transfer rate system is directly proportional
to the number of disks. The larger the number of physical disks striped together, the larger the
total transfer rate of the system. Hence, the overall performance and disk accessing speed is
also enhanced. The enhanced version of this scheme is mirroring or shadowing. In this RAID
organisation a duplicate copy of each disk is kept. It is costly but a much faster and more reliable
approach. The disadvantage with disk striping is that, if one of the N disks becomes damaged,
then the data on all N disks is lost. Thus striping needs to be combined with a reliable form of
backup in order to be successful.

Another RAID scheme uses some disk space for holding parity blocks. Suppose, three or more
disks are used, then one of the disks will act as a parity block, which contains corresponding bit
positions in all blocks. In case some error occurs or the disk develops a problems all its data bits
can be reconstructed. This technique is known as disk striping with parity or block interleaved
parity, which increases speed. But writing or updating any data on a disk requires corresponding
recalculations and changes in parity block. To overcome this the parity blocks can be distributed
over all disks.
RAID is a method of creating one or more pools of data storage space from several hard drives.
It can offer fault tolerance and higher throughput levels than a single hard drive or group of

independent hard drives. You can build a RAID configuration with IDE (parallel ATA), SATA

(Serial ATA) or SCSI hard disks or, in fact, even drives like the old 3.5” floppy disk drive!
The exact meaning of RAID has been much debated and much argued. The use of “Redundant”
is, in itself, a contentious point. That several manufacturers have deviated from accepted RAID
terminology, created new levels of disk arrangements, called them RAID, and christened them
with a number has not helped. There are even some single disk RAID confi gurations! Double
parity, RAID 1.5, Matrix RAID etc., are examples of proprietary RAID confi gurations.
Data can be distributed across a RAID “array” using either hardware, software or a combination of
the two. Hardware RAID is usually achieved either on-board on some server class motherboards
or via an add-on card, using an ISA/PCI slot.
Basic RAID levels are the building blocks of RAID. Compound RAID levels are built by using:
JBOD: JBOD is NOT RAID. JBOD stands for ‘Just a Bunch Of Disks’. This accurately describes
the underlying physical structure that all RAID structures rely upon. When a hardware RAID

controller is used, it normally defaults to JBOD configuration for attached disks. Some disk
controller manufacturers incorrectly use the term JBOD to refer to a Concatenated array.

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