Answer Posted / abhinav

RAID — which stands for Redundant Array of Inexpensive
Disks. A RAID distributes data across several physical disks
which look to the operating system and the user like a
single disk. Several different arrangements are possible. We
assume here that all the disks are of the same capacity, as
is usual.

We have only 3 basic RAID levels :

RAID 0: Provides improved performance and additional storage
but no fault tolerance. Any disk failure destroys the array,
which becomes more likely with more disks in the array. A
single disk failure destroys the entire array because when
data is written to a RAID 0 drive, the data is broken into
fragments. The number of fragments is dictated by the number
of disks in the drive. The fragments are written to their
respective disks simultaneously on the same sector. This
allows smaller sections of the entire chunk of data to be
read off the drive in parallel, giving this type of
arrangement huge bandwidth. RAID 0 does not implement error
checking so any error is unrecoverable. More disks in the
array means higher bandwidth, but greater risk of data loss.

RAID 1: Mirrored set without parity. Provides fault
tolerance from disk errors and failure of all but one of the
drives. Increased read performance occurs when using a
multi-threaded operating system that supports split seeks,
very small performance reduction when writing. Array
continues to operate so long as at least one drive is
functioning. Using RAID 1 with a separate controller for
each disk is sometimes called duplexing.

RAID 5: Striped set with distributed parity. Distributed
parity requires all drives but one to be present to operate;
drive failure requires replacement, but the array is not
destroyed by a single drive failure. Upon drive failure, any
subsequent reads can be calculated from the distributed
parity such that the drive failure is masked from the end
user. The array will have data loss in the event of a second
drive failure and is vulnerable until the data that was on
the failed drive is rebuilt onto a replacement drive.

Rest of RAID levels are the combination of these basic raid
levels.

which is better RAID 0+1 OR RAID 5? ...........

good question.......

RAID 0+1: striped sets in a mirrored set (minimum four
disks; even number of disks) provides fault tolerance and
improved performance but increases complexity.

Hence, Raid 5 is definitely better because if there is a
failure of even 1 disk in RAID 0+1.... the situation is same
as it is in raid 5 but, the min. no. of disks utilized in
raid 0+1 is 4 and on the other hand in raid 5, the similar
fault tolerance is given with a min of 3 HDDs.

Seniors kindly correct me if I am wrong.

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