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RAID: Redundant Array ofIndependent Disks

The idea was to combine multiple small, inexpensive physical disks into an array that would function as a single

logical drive, but provide better performance and higher data availability than a single large expensive disk drive

(SLED).

A study defined five different disk array configurations, or RAID levels. All of the RAID levels provided fault

tolerance, and each RAID level offered different feature sets and performance to accommodate different system

administration priorities and computing environments.

Common RAID levels are listed below:

RAID 0 (block-levelstripingwithoutparityormirroring)

RAID 1 (mirroring without parity or striping)

RAID 1+0 (Drive Mirroring and Striping)

RAID 2 (bit-level striping with dedicated Hamming-code parity)

RAID 3 (byte-level striping with dedicated parity)

RAID 4 (block-level striping with dedicated parity)

RAID 5 (block-level striping with distributed parity)

RAID 6 (block-level striping with double distributed parity)

RAID has three main attributes that are exploited(used at greatest extent) in some way by all original RAID

configurations and by most other RAID configurations that have been defined since the 1987study.

These attributes are:

A set of physical disk drives that can function as one or more logical drives (improved I/O)

Data distribution across multiple physical disks (striping)

Data recovery, or reconstruction of data in the event of a physical disk failure (redundancy)

Note: RAID 0 does not have all of these attributes.

RAID 2, RAID 3 and RAID 4 have become impractical due to technological changes. Other RAIDconfigurations (including some that are proprietary) have been defined over the years as well.
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RAID 0 (block-levelstripingwithoutparityormirroring)

RAID0 is simply data striped over several disks. This gives a performance advantage, as it is possible to read parts

of a file in parallel. However not only is there no data protection, it is actually less reliable than a single disk, as all

the data is lost if a single disk in the array stripe fails.

Minimum number of drives: 2

Refer the below URL for Animated Image which explains how RAID 0 works

http://www.lascon.co.uk/raid0.gif

The advantages of RAID 0 are as follows:

Highest performance configuration for writes

Lowest cost per unit of data stored All disk capacity is used to store data (none needed for fault tolerance)

The disadvantages of RAID 0 are as follows: All data on the logical drive is lost if a physical disk fails.

Online spare disks are not available.

Data preservation by backing up to external physical disks only.

Application: Provides high performance for reads and writes. However, there is no data redundancy. RAID 0 by

itself should only be used for applications that can tolerate loss of access to data and data that can reproduced

from other sources.

RAID 1 (mirroring without parity or striping) or simply called Disk mirroring

RAID1 is data mirroring. Two copies of the data are held on two physical disks, and the data is always identical.

RAID1 has a performance advantage, as reads can come from either disk, and is simple to implement. However, it is

expensive, as twice as many disks are needed to store the data.

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No data loss orinterruption of service if a disk fails.

Fast read performance data is available from either disk.

The disadvantages of RAID 1 are as follows:

High cost 50% of disk space is allocated for data protection, so only 50% of total disk

drive capacity is usable for data storage.

Application: Read intensiveOLTPand other transactional data for high performance and high availability. Other

applications that can benefit from RAID 1 include email, operating systems, application files, and read and random

intensive environments.

RAID 5 (block-level striping with distributed parity)

RAID 5 is the most common secure RAID level. The datas are transferred to disks by independent read and write

operations (not in parallel). 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. When a physical disk

fails, the data that was on the failed disk can be calculated from the parity data in the data blocks on the remaining

physical disks in the logical drive. Often extra cache memory is used on these controllers to improve the write

performance.

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High read performance

No loss of data if one physical disk fails.

More usable disk capacity than with RAID 1+0; parity information only requires the storage

space equivalent to one physical disk on the array.

The disadvantages of RAID 5 are as follows: Relatively low write performance

Data loss occurs if a second disk fails before data from the first failed disk is rebuilt.



Application: Reduces the number of components required while providing good availability, good performance for

reads, write performance is impacted if write cache not utilized. Good applications forRAID 5include reference

data, read intensive database tables, general file sharing and Web applications.

RAID 0+1 and RAID 1+0 (RAID 10)

It is quite common to combine the characteristics of RAID 0 and RAID 1 by creating two mirrors that are both

striped (RAID 0+1) or two mirrored sets with data striped between them (RAID 10). In RAID 0+1, the array will

continue to operate if one or more drives in the same mirror set fail. But, but if drives on both mirror sets fail, all the

data is lost. If disks fail, RAID 1+0 performs better than RAID 0+1 because all the remaining disks continue in use.

The array can suffer multiple drive failures as long as no mirror set loses all drives.

The advantages of RAID 1+0 are as follows:

Highest read and write performance of any fault-tolerant configuration. No loss of data as long as no of failed disks are mirrored to any other failed disk (up to half

of the physical disks in the array can fail).

The disadvantages of RAID 1+0 are as follows:

Expensive many disks are needed for fault tolerance.

Only 50% of total disk capacity usable for data storage.
http://www.lascon.co.uk/raid5.gifhttp://www.lascon.co.uk/raid5.gifhttp://searchsecurity.techtarget.com/answer/What-should-be-done-with-a-RAID-5-arrays-failed-driveshttp://searchsecurity.techtarget.com/answer/What-should-be-done-with-a-RAID-5-arrays-failed-driveshttp://searchsecurity.techtarget.com/answer/What-should-be-done-with-a-RAID-5-arrays-failed-driveshttp://searchsecurity.techtarget.com/answer/What-should-be-done-with-a-RAID-5-arrays-failed-driveshttp://www.lascon.co.uk/raid5.gif

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Application: OLTP and I/O-intensive applications requiring high performance and availability. This includes

transaction logs, journal files and database indices where the cost measurement is based on dollar per I/O,

compared to dollar per unit of storage

RAID striping

Very simply, RAID striping is a means of improving the performance of large storage systems. For most normal PCs

or laptops, files are stored in their entirety on a single disk drive, so a file must be read from start to finish and

passed to the host system. With large storage arrays, disks are often organized into RAID groups that can enhance

performance and protect data against disk failures. Striping is actually RAID-0; a technique that breaks up a file and

interleaves its contents across all of the disks in the RAID group. This allows multiple disks to access the contents of

a file simultaneously.

Instead of a single disk reading a file from start to finish, striping allows one disk to read the next stripe while the

previous disk is passing its stripe data to the host system -- this enhances the overall disk system performance,

which is very beneficial for busy storage arrays.

Parity can be added to protect the striped data. Parity data is calculated for the stripes and placed on another disk

drive. If one of the disks in the RAID group fails, the parity data can be used to rebuild the failed disk. However,

multiple simultaneous disk failures may result in data loss because conventional parity only accommodates a single

disk failure.

When to Use RAID 5

RAID 5 is favored for messaging, data mining, medium-performance media serving, and RDBMS implementations in which the

DBA is effectively using read-ahead and write-behind. If the host OS and HBA are capable of greater than 64 KB transfers, RAID5 is a compelling choice.

These application types are ideal for RAID 5:

1) Random workloads with modest IOPS-per-gigabyte requirements

2) High performance random I/O where writes represent 30 percent or less of the workload

3) A DSS database in which access is sequential (performing statistical analysis on sales records)

4) Any RDBMS table space where record size is larger than 64 KB and access is random (personnel records with binary content,

such asphotographs)5) RDBMS log activity

6) Messaging applications

7) Video/Media

When to Use RAID 1/0RAID 1/0 can outperform RAID 5 in workloads that use very small, random, and write-intensive I/Owhere more than 30

percent of the workload is random writes. Some examples of random, small I/O workloads are:

1) High-transaction-rate OLTP

2) Large messaging installations

3) Real-time data/brokerage records

4) RDBMS data tables containing small records that are updated frequently (account balances)

5) If random write performance is the paramount concern, RAID 1/0 should be used for these applications

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