Redundant Arrays of Inexpensive Disks - RAID 0-1-2-3 (Continued)
Descriptions of RAID 0:
RAID 0 known as data striping, distributes data across drives which results in higher data throughout.
Striping is optimized for performance at the expense of fault tolerance. Drives in RAID 0 array, mirrors the data across
the multiple drives. A RAID Level 0 array can obtain numerous stripes. In RAID 0 if 2 x 250 GB drives the array size will be
500 GB .
Advantages of RAID 0
The reason RAID 0 is a performance-enhancing configuration is that striping enables the array to directly access data from
the multiple drives simultaneously. As the data is spread out across a number of drives in the array, it can be accessed faster
without having "bottled-up neck" effect on a single drive. This is highly advantageous when retrieving large files, as the file
data is spread out effectively across multiple drives with direct accessibility yet treated as fragments on the data stripes.
Disadvantage of RAID 0
The fault tolerance is compromised; data loss is higher as there is no space made available to store the redundant data.
Any drive failure in the RAID 0, data loss is imminent.
Descriptions of RAID 1:
The RAID 1 known as drive mirroring, works by simultaneously copying data to a second drive so no data is lost if there is
drive failure, it has enhanced reliability achieved by duplicating/mirroring all the data on the available drives.
RAID 1 enhanced performance depends upon available capacity in the drives.
In RAID 1 if you have 2 x 250 GB drives the array size will be 250 GB.
Advantages of RAID 1
This provides a full level of redundancy for the data on the system as the data is mirrored on to both drives.
If one of the drives fails, the other drive still has all the data that existed in the system.
Disadvantage of RAID 1
The RAID 1 capacity will only be as big the smallest of the drives, effectively halving the amount of storage capacity if
the two drives were used. if you have 2 x 250 GB drives the array size will be 250 GB. The number of read requests and total
wait-state times both drop significantly; inversely proportional to the number of hard drives in the RAID.
Descriptions of RAID 2:
RAID 2 system provides protection in case hard drives in the subsystem incur problems or otherwise fail.
Advantages of RAID 2
This level builds fault tolerance based on Hamming Error Correction Code (ECC), which is used to maintain the integrity of data.
Consequently, the higher rate of data transfer required, the better the ratio of data disks to ECC disks.
If the numbers match, the data is intact; if there is a discrepancy, the lost data can be recalculated using the first or
earlier checksum as a reference point. Subsequently speedy data error correction. Relatively, the controller design is simpler
than that of RAID levels 3,4 & 5.
Disadvantage of RAID 2
Commercially is unviable as it is inefficient, transaction data rate is at the rate of a single disk at best
(with spindle synchronization). Entry level cost very high - requires very high transfer data rate requirement to justify
very high ratio of ECC disks to data disks.
The data block is subdivided "striped" and written on the
data disks. Stripe parity is generated on Writes, recorded on
the parity disk and checked on Reads. Stripes data at a byte level across several drives, with parity stored on one drive.
It is otherwise similar to level 4. Byte-level striping requires hardware support for the most efficient use.
Continue to Raid Levels 4,5,6