Using nvSRAM in RAID Controller Applications
Level 4 - Independent Access Array
Level 5 - Independent Access Array with Rotating . .
Parity
RAID Level 3
RAID 3 is optimized for high data transfer rates and
is a parallel transfer technique with parity. Each data
sector is subdivided, and data is scattered across all
data disks with redundant data being stored on a
dedicated parity disk. Reliability is much higher than
a single disk and the data transfer capacity is the
highest of all listed RAID types. RAID 3’s weakness
lies in its relatively slow I/O rates that make it unsuit-
able for most transaction processing unless assisted
by some other technology such as cache. The parity
disk stores redundant information about the data
chunks stored in corresponding locations on the
data disks. The redundant information is typically in
the form of a bit-by-bit Exclusive OR function of cor-
responding data chunks from the other disks. Typi-
cal applications for RAID 3 include large data
objects such as CAD files, graphical images, seis-
mic or telemetered data streams.
Level 6 - Recovery from the failure of up to 2 disks
RAID Level 0
A stripe set presents a single virtual disk whose
capacity is equal to the sum of the capacities of its
members. The reliability of the stripe is less than the
reliability of its least reliable member and its read
and write rates are high. RAID 0 is not a true RAID
controller because it provides no redundancy. It is,
however, a performance-oriented architecture that is
inexpensive and therefore attractive to many low
cost users. RAID 0 is a parallel transfer technology.
RAID Level 1
A mirror set also presents a single virtual disk; its
capacity however is equal to that of its smallest
member. Its reliability is very high, its read perfor-
mance is usually better than that of a single mem-
ber, but its write performance is somewhat slower. A
RAID 1 system protects against disk failure by repli-
cating all stored data at least once on a physically
separate disk. RAID 1 can be implemented as either
a parallel or independent array and is well suited to
applications that are read intensive and where reli-
ability requirements are high.
RAID Level 4
RAID level 4 is an independent access array in
which data sectors are distributed in a similar man-
ner to disk striping systems. Redundant data is
stored on an independent parity disk (similar to
RAID 3). Its data reliability is much higher than a sin-
gle disk (comparable to RAID 2, 3, and 5) and its
data transfer capacity is moderate. RAID 4 is a high
I/O read rate technology with moderate write
speeds, but is not well suited for high data transfer
applications due to the parity disk write bottleneck
Two of the four operations required to perform a vir-
tual disk write are directed at the parity disk; for this
reason RAID 4 arrays are seldom implemented.
Possible applications would include systems that
are read intensive and do not require high data
transfer rates.
RAID Level 2
A parallel access array that uses Hamming Coding
to provide error detection and correction capability
to the array. This approach is very expensive and
therefore almost never implemented into a system.
Virtual Disk
RAID Level 5
RAID level 5 is an independent access array with
rotating parity. Data sectors are distributed in the
same manner as disk striping systems but redun-
dant information is interspersed with user data
across multiple array members rather than stored
on a single parity disk as in RAID 3/4 systems. This
relieves the write bottleneck associated with RAID
level 4 controllers. RAID 5 arrays have high data
reliability, good data transfer rates and high I/O rate
capability. It is well suited to applications such as on-
line customer services, inquiry-type transaction pro-
cessing, group office automation, etc.
Array Management Software
Data Disk
Data Disk
Data Disk
Data Disk
Parity Disk
Figure 2
Example of a Typical RAID Level 3 or 4 Controller
From The Roadblock Edition 1-1
8-26
SIMTEK [ SIMTEK CORPORATION ]
