RAID is a technology that is used to increase the performance and/or reliability of data storage. The abbreviation stands for either Redundant Array of Independent Drives or Redundant Array of Inexpensive Disks, which is older and less used. A RAID system consists of two or more drives working in parallel. These can be hard discs, but there is a trend to also use the technology for SSD (Solid State Drives).
There are different RAID levels, each optimized for a specific situation. This explains why companies sometimes come up with their own unique numbers and implementations.
This article covers most used RAID levels:
RAID 0 – striping
RAID 1 – mirroring
RAID 5 – striping with parity
RAID 6 – striping with double parity
RAID 10 – combining mirroring and striping
The software to perform the RAID-functionality and control the drives can either be located on a separate controller card (a hardware RAID controller) or it can simply be a driver.
Sometimes disks in a storage system are defined as JBOD, which stands for Just a Bunch Of Disks. This means that those disks do not use a specific RAID level and acts as stand-alone disks. This is often done for drives that contain swap files or spooling data.
Below is an overview of the most popular RAID levels:
RAID level 0 – Striping
In a RAID 0 system data are split up into 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 further enhanced by using multiple controllers, ideally one controller per disk.
RAID 0 – Striping
Advantages of RAID 0
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 overhead.
The technology is easy to implement.
Disadvantages of RAID 0
RAID 0 is not fault-tolerant. If one drive fails, all data in the RAID 0 array are lost. It should not be used for mission-critical systems.
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 an image retouching or video editing station.
If you want to use RAID 0 purely to combine the storage capacity of two drives in a single volume, consider mounting one drive in the folder path of the other drive. This is supported in Linux, OS X as well as Windows and has the advantage that a single drive failure has no impact on the data of the second disk or SSD drive.
RAID level 1 – Mirroring
Data are stored twice by writing them to both the data drive (or set of data drives) and a mirror drive (or set of drives). If a drive fails, the controller uses either the data drive or the mirror drive for data recovery and continuous operation. You need at least 2 drives for a RAID 1 array.
RAID 1 – Mirroring
Advantages of RAID 1
RAID 1 offers excellent read speed and a write-speed that is comparable to that of a single drive.
In case a drive fails, data do not have to be reconstructed, they just have to be copied to the replacement drive.
RAID 1 is a very simple technology.
Disadvantages of RAID 1
The main disadvantage is that the effective storage capacity is only half of the total drive capacity because all data gets written twice.
Software RAID 1 solutions do not always allow a hot swap of a failed drive. That means the failed drive can only be replaced after powering down the computer it is attached to. For servers that are used simultaneously by many people, this may not be acceptable. Such systems typically use hardware controllers that do support hot swapping.
Ideal use
RAID-1 is ideal for mission critical storage, for instance for accounting systems. It is also suitable for small servers in which only two data drives will be used.
RAID level 5 – Striping with parity
RAID 5 is the most common secure RAID level. It requires at least 3 drives but can work with up to 16. Data blocks are striped across the drives and on one drive a parity checksum of all the block data is written. The parity data are not written to a fixed drive, they are spread across all drives, as the drawing below shows. Using the parity data, the computer can recalculate the data of one of the other data blocks, should those data no longer be available. That means a RAID 5 array can withstand a single drive 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.
RAID 5 – Striping with parity
Advantages of RAID 5
Read data transactions are very fast while write data transactions are somewhat slower (due to the parity that has to be calculated).
If a drive fails, you still have access to all data, even while the failed drive is being replaced and the storage controller rebuilds the data on the new