RAID

RAID (redundant array of independent disks, originally redundant array of inexpensive

disks) is a storage technology that combines multiple disk drive components into a logical unit.
Data is distributed across the drives in one of several ways called "RAID levels",depending on
what level of redundancy and performance (via parallel communication) is required.

RAID is now used as an umbrella term for computer data storage schemes that can divide and
replicate data among multiple physical drives. The physical drives are said to be "in a RAID",he
different schemes or architectures are named by the word RAID followed by a number (e.g.,
RAID 0, RAID 1). Each scheme provides a different balance between three key goals:
resiliency, performance, and capacity.

The standard RAID levels are a basic set of RAID configurations and employ striping,
mirroring, or parity.

RAID level 0 – Striping

A RAID 0 (also known as a stripe set or striped volume) splits data evenly across two or more
disks(striped) without parity information for speed. RAID 0 was not one of the original RAID
levels and provides no data redundancy. RAID 0 is normally used to increase performance,
although it can also be used as a way to create a large logical disk out of two or more physical
ones.

A RAID 0 can be created with disks of differing sizes, but the storage space added to the array
by each disk is limited to the size of the smallest disk. For example, if a 100 GB disk is striped
together with a 350 GB disk, the size of the array will be 200 GB (100 GB x 2).

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RAID Level 0 requires a minimum of 2 drives to implement.

Advantages

 No parity generation
 Easy to implement
 Cost effective
 Utilizes full disk capacity
 RAID 0 offers great performance, both in read and writes operations. There is no
overhead caused by parity controls.
 All storage capacity is used, there is no disk overhead.
 The technology is easy to implement.

Disadvantages

RAID 0 is not fault-tolerant. If one disk fails, all data in the RAID 0 array are lost. It should not
be used on mission-critical systems.

 Not a true RAID

 No redundancy / fault tolerance
 Drive failure will result in data loss
 Not suitable in mission critical environments

Ideal use

RAID 0 is ideal for non-critical storage of data that have to be read/written at a high speed, such
as on a Photoshop image retouching station.

 Pre-Press
 Video editing and production
 Image manipulation/editing

RAID level 1 – Mirroring

An exact copy (or mirror) of a set of data on two disks. This is useful when read performance or
reliability is more important than data storage capacity. Such an array can only be as big as the
smallest member disk. A classic RAID 1 mirrored pair contains two disks.

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Provides redundancy by writing all data to two or more drives. The performance of a level 1
array tends to be faster on reads and slower on writes compared to a single drive, but if either
drive fails, no data is lost. This is a good entry-level redundant system, since only two drives are
required; however, since one drive is used to store a duplicate of the data, the cost per megabyte
is high. This level is commonly referred to as mirroring. RAID Level 1 requires a minimum of 2
drives to implement

Advantages

 No parity generation

 Easy to implement

 Extremely fault tolerant

 Utilizes full disk capacity

 2 drives minimum

100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to
the replacement disk, Simplest RAID storage subsystem design

Disadvantages

The main disadvantage is that the effective storage capacity is only half of the total disk capacity
because all data get written twice.

 Inefficient use of disk space

 High disk overhead

 Doubles number of writes

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Typically the RAID function is done by system software, loading the CPU/Server and possibly
degrading throughput at high activity levels. Hardware implementation is strongly
recommended. May not support hot swap of failed disk when implemented in "software"

Recommended Applications

 Accounting

 Payroll

 Financial

 Any application requiring very high availability

RAID level 3

In RAID 3 (byte-level striping with dedicated parity), all disk spindle rotation is synchronized,
and 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. On RAID 3 systems, data blocks are
subdivided (striped) and written in parallel on two or more drives. An additional drive stores
parity information. You need at least 3 disks for a RAID 3 array.

Since parity is used, a RAID 3 stripe set can withstand a single disk failure without losing data
or access to data.

Advantages

 RAID-3 provides high throughput (both read and write) for large data transfers.

 Disk failures do not significantly slow down throughput.

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Disadvantages

 This technology is fairly complex and too resource intensive to be done in software.

 Performance is slower for random, small I/O operations.

Inefficient with small file transfer

Recommended Applications

 Video Production and live streaming

 Image Editing
 Video Editing
 Any application requiring high throughput

RAID level 5 (block-level striping with distributed parity)

RAID 5 is the most common secure RAID level. It is similar to RAID-3 except that data are
transferred to disks by independent read and write operations (not in parallel). The data chunks
that are written are also larger. Instead of a dedicated parity disk, parity information is spread
across all the drives. You need at least 3 disks for a RAID 5 array.

A RAID 5 array can withstand a single disk failure without losing data or access to data.
Although RAID 5 can be achieved in software, a hardware controller is recommended. Often
extra cache memory is used on these controllers to improve the write performance.

A RAID 5 uses block-level striping with parity data distributed across all member disks. RAID 5
has achieved popularity because of its low cost of redundancy. This can be seen by comparing

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the number of drives needed to achieve a given capacity. For an array of drives, with
being the size of the smallest disk in the array, other RAID levels that yield redundancy give
only a storage capacity of (for RAID 1), or (for RAID 1+0). In RAID 5,
the yield is . For example, four 1 TB drives can be made into two separate
1 TB redundant arrays under RAID 1 or 2 TB under RAID 1+0, but the same four drives can be
used to build a 3 TB array under RAID 5.

RAID 5 requires at least three disks. Provides redundancy by writing data and parity information
across three or more drives, thus increasing performance. The RAID Level 5provides the best
combination of disk array technology. The most popular RAID level supplied.

Characteristics & Advantages

 Highest Read data transaction rate

 Medium Write data transaction rate

 Low ratio of ECC (Parity) disks to data disks means high efficiency

 Good aggregate transfer rate

 Ideal for small write applications

 Highly efficient
 Read data transactions are very fast while write data transaction are somewhat slower
(due to the parity that has to be calculated).

Disadvantages

 Disk failures have an effect on throughput, although this is still acceptable.

 Like RAID 3, this is complex technology.

Ideal use

RAID 5 is a good all-round system that combines efficient storage with excellent security and
decent performance. It is ideal for file and application servers.

 File and Application servers

 Database servers
 Web, E-mail, and News servers
 Intranet servers
 Most versatile RAID level

RAID level 10 – Combining RAID 0 & RAID 1

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In RAID 10 (mirroring and striping), data is written in stripes across the primary disks and then
mirrored to the secondary disks. A typical RAID 10 configuration consists of four drives. Two
for striping and two for mirroring. A RAID 10 configuration takes the best concepts of RAID 0
and RAID 1 and combines them to provide better performance along with the reliability of parity
without actually having parity as with RAID 5 and RAID 6. RAID 10 is often referred to as
RAID 1+0 (mirrored+striped).

RAID 10 combines the advantages (and disadvantages) of RAID 0 and RAID 1 in one single
system. It provides security by mirroring all data on a secondary set of disks (disk 3 and 4 in the
drawing below) while using striping across each set of disks to speed up data transfers.

Advantages

 Highly fault tolerant

 High data availability

 Very good read / write performance

High I/O rates are achieved by striping RAID 1 segments

Disadvantages

 Very expensive

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  Drive spindles must be synchronised

 Not very scalable

Recommended Applications

 Database server requiring high performance and fault tolerance

All RAID levels except RAID 0 offer protection from a single drive failure. A RAID 6 system
even survives 2 disks dying simultaneously. For complete security you do still need to back-up
the data from a RAID system.

RAID 6

 Just like RAID 5, this does block level striping. However, it uses dual parity.

 In the above diagram A, B, C are blocks. p1, p2, p3 are parities.

 This creates two parity blocks for each data block.

 Can handle two disk failure

 This RAID configuration is complex to implement in a RAID controller, as it has to

calculate two parity data for each data block.

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RAID is a method of combining multiple disk drives into a single entity in order to improve the
overall performance and reliability of your system. The different options for combining the disks
are referred to as RAID levels. There are several different levels of RAID available depending on
the needs of your system. One of the options available to you is whether you should use a
Hardware RAID solution or a Software RAID solution.

RAID Hardware is always a disk controller to which you can cable up the disk drives. RAID
Software is a set of kernel modules coupled together with management utilities that implement
RAID in Software and require no additional hardware.

Pros and cons

Software RAID is more flexible than Hardware RAID. Software RAID is also considerably less
expensive. On the other hand, a Software RAID system requires more CPU cycles and power to
run well than a comparable Hardware RAID System. Also, because Software RAID operates on
a partition by partition basis where a number of individual disk partitions are grouped together as
opposed to Hardware RAID systems which generally group together entire disk drives, Software
RAID tends be slightly more complicated to run. This is because it has more available
configurations and options. An added benefit to the slightly more expensive Hardware RAID
solution is that many Hardware RAID systems incorporate features that are specialized for
optimizing the performance of your system

Summary:

1. Unlike software RAID, Hardware RAID requires specialized hardware to handle the drives.

2. Software RAID is considerably cheaper than hardware RAID.

3. Unlike hardware RAID, Software RAID takes up a portion of the host processor.

4. Hardware RAID is more reliable compared to software RAID.

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magneto-optical drive (MO)
A rewritable optical disk that is used in combination with magnetic technology. An MO system
achieves its high data density by using a laser and a magnetic read/write head in combination.
Both the laser and the magnet are used to write data onto the diskette. The laser heats up the
diskette surface so it can be easily magnetized, and also to allow the region of magnetization to
be precisely located and confined. A less intense laser is used to read data from the diskette. Data
can be erased and/or overwritten an unlimited number of times, as with a conventional3.5-inch
diskette.

Laser and Magnetic Writing-Data are written on an MO disk by a laser and a magnet. The
laser heats the bit to the Curie point, which is the temperature at which molecules can be
realigned when subjected to a magnetic field. Then, a magnet changes the bit's polarity. The laser
is focused on one side of the platter, and the magnet is used on the opposite side, which is why
double sided media must be flipped over to access the other side.

Laser Reading-Reading is accomplished with a lower-power laser that reflects light from the
bits. The light is rotated differently depending on the polarity of the bit, and the difference in
rotation is sensed. Writing takes two passes. The existing bits are set to zero in one pass, and data
are written on the second pass. A direct overwrite method (LIMDOW) was later added that
erases and writes in one rotation. Many drives support the LIMDOW disks, which is more costly
than standard MO media.

Examples of magneto-optical drives are the Fujitsu DynaMO, a 230 MB drive used in the
PowerPC Apple Powerbook, a note book computer, and the Pinnacle Micro Vertex, a 2.6 GB
drive.

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BLU-RAY DISC

Blu-ray Disc (official abbreviation BD) is an optical disc storage medium designed to supersede
the standard DVD format. Its main uses are for storing high-definition video, PlayStation 3 video
games, and other data, with up to 25 GB per single-layered, and 50 GB per dual-layered disc. ...

While current optical disc technologies such as DVD, DVD±R, DVD±RW, and DVD-RAM rely
on a red laser to read and write data, the new format uses a blue-violet laser instead, hence the
name Blu-ray. Despite the different type of lasers used, Blu-ray products can easily be made
backwards compatible with CDs and DVDs through the use of a BD/DVD/CD compatible
optical pickup unit. The benefit of using a blue-violet laser (405nm) is that it has a shorter
wavelength than a red laser (650nm), which makes it possible to focus the laser spot with even
greater precision. This allows data to be packed more tightly and stored in less space, so it's
possible to fit more data on the disc even though it's the same size as a CD/DVD. This together
with the change of numerical aperture to 0.85 is what enables Blu-ray Discs to hold 25GB/50GB.
Recent development by Pioneer has pushed the storage capacity to 500GB on a single disc by
using 20 layers.

What Blu-ray formats are planned?

As with conventional CDs and DVDs, Blu-ray plans to provide a wide range of formats
including ROM/R/RW. The following formats are part of the Blu-ray Disc specification:

BD-ROM - read-only format for distribution of HD movies, games, software, etc.

BD-R - recordable format for HD video recording and PC data storage.
BD-RE - rewritable format for HD video recording and PC data storage.

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How much data can you fit on a Blu-ray disc?

A single-layer disc can hold 25GB.

A dual-layer disc can hold 50GB.

How fast can you read/write data on a Blu-ray disc?

According to the Blu-ray Disc specification, 1x speed is defined as 36Mbps. However, as BD-
ROM movies will require a 54Mbps data transfer rate the minimum speed we're expecting to see
is 2x (72Mbps). Blu-ray also has the potential for much higher speeds,

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