RAID (redundant array of independent disks; originally redundant array of inexpensive disks) is a way of storing the same data in different places (thus, redundantly) on multiple hard disks. By placing data on multiple disks, I/O operations can overlap in a balanced way, improving performance. Since multiple disks increases the mean time between failures, storing data redundantly also increases fault tolerance.
A RAID appears to the operating system to be a single logical hard disk. RAID employs the technique of disk striping, which involves partitioning each drive’s storage space into units ranging from a sector (512 bytes) up to several megabytes. The stripes of all the disks are interleaved and addressed in order.
In a single-user system where large records, such as medical or other scientific images, are stored, the stripes are typically set up to be small (perhaps 512 bytes) so that a single record spans all disks and can be accessed quickly by reading all disks at the same time.
In a multi-user system, better performance requires establishing a stripe wide enough to hold the typical or maximum size record. This allows overlapped disk I/O across drives.
There are types of RAID
RAID-0: The idea of RAID 0 is to increase performance. When storing information using the striping feature, the data will be split block by block between the two hard disks. Block one will be send to disk one, block two will be sent to disk two, block three will be sent to disk one and block 4 will be sent to disk 2 and so on. This is much faster than a single disk because when reading the data off the disks the twp of them will be working at the same time to retrieve the same file virtually doubling the speed or retrieval and so virtually halving the time of retrieval. As I mentioned this is a performance setup. Should any one of the disks fail the whole array will become corrupt. Most of the files will be split between disks and so will be rendered useless. If you don’t have important data on your computer or you have regular backups of what you do need, then a RAID 0 setup would greatly increase your computers disk performance. To get the best out of this system it is wise to use two disks which are the same make and model. If this is not possible then two of the same size and RPM would be useful but not essential. If two disks of different sizes are used in this system then the logical drive will show as the smallest disk. See drive capacities under RAID at the end of this article.
RAID-1: RAID 1 or mirroring gives added security for your data at the cost of storage space. As with striping this setup uses two hard disk drives to produce a single logical drive. In this instance however the total storage space is only the size of one of the disks (the smallest one). This is because with RAID 1 any data that is written to or read from the hard disk is done on the second hard disk exactly the same. If you save a file to your machine, it will will saved on both disks at the same time. This will however affect system performance with two disks needing to be written to and with the data being the same, its no better in terms of performance unlike the stripping method. However there is always advantages. Mirroring, having the same data on both disks has obvious plus points when it comes to data integrity and security against disk failure. If either disk one or two should fail the other disk will take over as the solitary disk providing and storing data like it did before the failure. Again see the drive capacity section at the end of this article to learn about data redundancy and why the logical drive sizes are what they are with each of the three RAID setups.
RAID-10:RAID 0+1 or Striping + Mirroring as you would imagine is a combination of the above two setups. This setup takes the advantages of both the stripping setup and the mirroring setup. You get the increased performance of splitting the data across multiple drives, however each of these striped drives will have a mirror as well for the data backup and security against failure. The obvious drawback here is the cost involved. The minimum amount of hard disk drives used in this configuration is 4. This puts most home users out of the equation as not only do you need to buy 4 hard disks but the PSU has to cope as well.
RAID-2: This type uses striping across disks with some disks storing error checking and correcting (ECC) information. It has no advantage over RAID-3.
RAID-3: This type uses striping and dedicates one drive to storing parity information. The embedded error checking (ECC) information is used to detect errors. Data recovery is accomplished by calculating the exclusive OR (XOR) of the information recorded on the other drives. Since an I/O operation addresses all drives at the same time, RAID-3 cannot overlap I/O. For this reason, RAID-3 is best for single-user systems with long record applications.
RAID-4: This type uses large stripes, which means you can read records from any single drive. This allows you to take advantage of overlapped I/O for read operations. Since all write operations have to update the parity drive, no I/O overlapping is possible. RAID-4 offers no advantage over RAID-5.
RAID-5: This type includes a rotating parity array, thus addressing the write limitation in RAID-4. Thus, all read and write operations can be overlapped. RAID-5 stores parity information but not redundant data (but parity information can be used to reconstruct data). RAID-5 requires at least three and usually five disks for the array. It’s best for multi-user systems in which performance is not critical or which do few write operations.
RAID-6: This type is similar to RAID-5 but includes a second parity scheme that is distributed across different drives and thus offers extremely high fault- and drive-failure tolerance.
RAID-7: This type includes a real-time embedded operating system as a controller, caching via a high-speed bus, and other characteristics of a stand-alone computer. One vendor offers this system.
RAID-50 (or RAID-5+0): This type consists of a series of RAID-5 groups and striped in RAID-0 fashion to improve RAID-5 performance without reducing data protection.
RAID-53 (or RAID-5+3): This type uses striping (in RAID-0 style) for RAID-3’s virtual disk blocks. This offers higher performance than RAID-3 but at much higher cost.
You can see how to do raid-01 technology with below pictures.
Disk Capacities Using the RAID Function
These RAID functions give you varying capacities for your hard disk. To illustrate this we will take an example of a user using only 80Gb hard disks. We will take each of RAID levels mentioned to see what drive capacities you would get out of them.
RAID 0 Striping
With RAID 0 and using the 2 80Gb hard disks you would get the full 160Gb of storage space. Although the data is split between the 2 hard disks. There is no data redundancy (duplicate data). This allows for the full storage space to be used.
RAID 1 Mirroring
When using two 80Gb hard disks with the RAID 1 function you would only receive 80Gb of storage space. Because you are using the two drives to contain the same data, the logical drive will appear as a single 80Gb drive.
RAID 0+1 Striping
In this example we would need to use 4 80Gb drives. RAID 0+1 is a combination of the two above and so storage works out as a combination of the two as well. The logical drive will appear as a single drive, this drives capacity will be 160Gb. The 2 striped drives will be included in the logical drives space, but as above the mirrored drives will appear invisible to the user.