RAID Levels

RAID levels

RAID level 0 – Striping

In a RAID 0 system data are split up in blocks that get written across all the drives in the array. By using multiple disks (at least 2) at the same time, this offers superior I/O performance. This performance can be enhanced further by using multiple controllers, ideally one controller per disk.

Advantages

• RAID 0 offers great performance, both in read and write 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.

Applications

• Pre-Press
• Video editing and production
• Image manipulation/editing
• Downloading 

RAID level 1 – Mirroring

Data are stored twice by writing them to both the data disk (or set of data disks) and a mirror disk (or set of disks) . If a disk fails, the controller uses either the data drive or the mirror drive for data recovery and continues operation. You need at least 2 disks for a RAID 1 array.

RAID 1 systems are often combined with RAID 0 to improve performance. Such a system is sometimes referred to by the combined number: a RAID 10 system.

Using RAID 1 with a separate controller for each disk is sometimes called
duplexing.

Advantages

• RAID 1 offers excellent read speed and a write-speed that is comparable to that of a single disk.
• In case a disk fails, data do not have to be rebuild, they just have to be copied to the replacement disk.
• RAID 1 is a very simple technology.

Disadvantages

• Inefficient use of disk space
• High disk overhead
• Doubles number of writes

Applications

Transaction,logging or record keeping applications 

RAID level 3

Byte-level striping with dedicated parity.

On RAID 3 systems, data blocks are subdivided (striped) and written in parallel on two or more drives. An additional drive stores parity information (X-OR). You need at least 3 disks for a RAID 3 array. It combines 5 or 9 disks.

Advantages

• RAID-3 provides high throughput (both read and write) for large data transfers.
• Disk failures do not significantly slow down throughput.

Disadvantages

• This technology is fairly complex and too resource intensive to be done in software.
• Performance is slower for random, small I/O operations.

Ideal use

RAID 3 is not that common in purpose.

RAID level4

 Block-level striping with dedicated parity.

RAID level 5

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.

Advantages

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.

RAID 6

Block-level striping with double distributed parity.

RAID 10 (or 1+0): Reliable, High Performing Mirrored Stripes
RAID 10 is a mirrored stripe. At its most simple, two disks are mirrored and then those two mirrors are striped together into one LUN, which is then presented out to a server. Due to the number of disks used, RAID 10 is a moderately high cost solution, however it does offer decent I/O rates.

Advantages

• Highly fault tolerant
• High data availability
• Very good read / write performance

Disadvantages

• Very expensive
• Drive spindles must be synchronised
• Not very scaleable

RAID 01 (or 0+1): High Performing Striped Mirrors
RAID 01 is a striped mirror. In its simplest form, two sets of two disks are striped, then then are mirrored. RAID 01 is an yields high performance but not maximum reliability.

Advantages

• No parity generation
• Easy to implement
• Utilises full disk capacity
• 4 drives minimum
• Higher performance than RAID 5

Disadvantages

• Inefficient use of disk space
• High disk overhead / Expensive
• Costly to deploy