Transcript RAID
Redundant Array of Independent Disks
Many systems today need to store many
terabytes of data.
Don’t want to use single, large disk
too expensive
failures could be catastrophic
Would prefer to use many smaller disks.
is a storage technology.
was first defined by David Patterson, Garth A.
Gibson, and Randy Katz at the University of
California, Berkeley in 1987.
is the organization of multiple disks into a
large, high performance logical disk.
An array of multiple disks accessed
in parallel will give greater throughput than a
single disk.
Redundant data on multiple disks
provides fault tolerance.
Striping
Redundancy
Take file data and map it to different disks
Allows for reading data in parallel
file data
block 0
Disk 0
block 1
Disk 1
block 2
Disk 2
block 3
Disk 3
In engineering, redundancy is the duplication
of critical components or functions of a
system with the intention of increasing
reliability of the system, usually in the case of
a backup or fail-safe.
Data redundancy occurs in database
systems which have a data that is repeated in
two or more disks.
A number of standard schemes have evolved
which are referred to as levels.
There were five RAID levels originally
conceived
Other kinds have been proposed in literature
Level 2 and 4 are not commercially available
Break a file into blocks of data
Stripe the blocks across disks in the system
provides no redundancy or error detection
important to consider because lots of disks means
low Mean Time To Failure (MTTF)
A complete file is stored on a single disk
A second disk contains an exact copy of the file
Provides complete redundancy of data
Most expensive RAID implementation
requires twice as much storage space
RAID 2 implements bit striping with ECC
Error correction code (Hamming code) allows
for correction of a single bit error
is not as efficient as other RAID levels and is
not generally used.
Data is striped so each sequential byte is on a
different drive
Parity is calculated across corresponding
bytes and stored on a dedicated parity drive.
It requires only one disk for parity data.
RAID 3 suffers from a write bottleneck.
Similar to RAID 3.
It employs striped data in much larger blocks
or segments.
Not used commercially.
Distribution of the parity strip to avoid the
bottle neck.
Best of all worlds
read and write performance close to that of RAID Level-1
requires as much disk space as Levels-3,4
Combine two levels and get the advantages
from both.
Examples: 0+1, 1+0, 0+3, 3+0, 0+5, 5+0, 1+5,
and 5+1.
Today, RAID is found everywhere--In operating system software.
A stand-alone controller providing advanced
data integrity in high-end storage area
networks.
Laptops, as well as desktops, workstations,
servers, and external enclosures with a larger
number of hard disk drives.
RAID is even included in TV set top boxes or
personal storage devices.