RAID (Redundant Arrays of Independent Disks) uses multiple disk drives together to improve performance and reliability. It introduces redundancy through techniques like disk mirroring and parity bits to protect against data loss from disk failures. The document discusses various RAID levels that offer different combinations of redundancy, performance, and cost tradeoffs such as striping data across disks for faster read/write speeds and storing parity on separate disks. Choosing an appropriate RAID level depends on factors like storage costs, I/O performance needs, and data availability during disk failures or rebuild processes.

1. RAID
A variety of disk-organization techniques, collectively called redundant arrays of independent disks (RAID),
have been proposed to achieve improved performance and reliability
Improved reliability via redundancy:
 Measured of the reliability of the disk is MTTF (Mean Time to Failure) which states that the amount of
time that, on average, we can expect the system to run continuously without any failure.
 The chance that some disk out of a set of N disks will fail is much higher than the chance that a specific
disk will fail.
Suppose that the MTTF of a disk is 100,000 hours or slightly over 11 years
MTTF of some disk in an array of 100 disks will be 100,000/100 = 1000 hours or around 42 days, which
is not long at all!
 If it is stored only one copy of data, then each disk failure will result in loss of a significant amount of
data. Such a high rate of data loss is unacceptable. The solution is to introduce redundancy – store
extra information that is not needed normally, but that can be used in the event of failure of a disk to
rebuild the lost information.
 Approaches: The simplest but most expensive approach to introduce redundancy is to duplicate every
disk. This technique is called mirroring or shadowing.
Improved performance via parallelism:
1. With disk mirroring, the rate at which read requests can be handled is doubled, since read requests
can be sent to either disk
2. The transfer rate of each read is the same as in a single disk system, but the number of reads per unit
time is doubled.
3. With multiple disks we can improve the transfer rate by striping data (bit level striping or block level
striping) across multiple disks.
 In its simplest form, data striping consists of splitting the bits of each byte across multiple disks;
such striping is called bit-level striping
 Block-level striping stripes blocks across multiple disks. It treats the array of disks
as a single large disk, and it gives blocks logical numbers; we assume the block numbers start from
0.

2. RAID Levels:
 Mirroring provides high reliability but it is expensive. Striping provides high data transfer rates, but does
not improve reliability. So it requires redundancy at lower cost.
 Various alternative schemes aim to provide redundancy at lower cost by combining disk striping with
parity bits. These schemes have different cost-performance trade-offs. These schemes are classified
into RAID levels as in Figure 1. (In the figure, P indicates error-correcting bits, and C indicates a second
copy of the data.) For all levels, the figure depicts four disk’s worth of data, and the extra disks depicted
are used to store redundant information for failure recovery.
• RAID level 0 refers to disk arrays with striping at the level of blocks, but without any redundancy (such
as mirroring or parity bits).
• RAID level 1 refers to disk mirroring with block striping.
• RAID level 2, known as memory-style error-correcting-code (ECC) organization, employs parity bits.
Memory systems have long used parity bits for error detection and correction.
RAID level 3, bit-interleaved parity organization, improves on level 2 by exploiting the fact that disk
controllers, unlike memory systems, can detect whether a sector has been read correctly, so a single
parity bit can be used for error correction, as well as for detection.

3. RAID level 4, block-interleaved parity organization, uses block level striping, like RAID 0, and in
addition keeps a parity block on a separate disk for corresponding blocks from N other disks.
RAID level 5, block-interleaved distributed parity, improves on level 4 by partitioning data and parity
among all N + 1 disks, instead of storing data in N disks and parity in one disk.
RAID level 6, the P + Q redundancy scheme, is much like RAID level 5, but stores extra redundant
information to guard against multiple disk failures. Instead of using parity, level 6 uses error-correcting
codes such as the Reed–Solomon codes
Choice of RAID Level
The factors to be taken into account when choosing a RAID level are
• Monetary cost of extra disk storage requirements
• Performance requirements in terms of number of I/O operations
• Performance when a disk has failed
• Performance during rebuild (that is, while the data in a failed disk is being rebuilt on a new disk)