RAID, or Redundant Array of Independent Disks, is effectively a number of hard drives that work together to either optimise your server for performance, for reliability or for both.
There are seven commonly used RAID systems, listed below;
RAID 0 is, in a nutshell, purely about performance. Through a process called striping, data is broken up and written across multiple drives meaning that large amounts of data can be processed in the minimum possible time. The issue with RAID 0 is that if one hard drive fails, all your data is lost due the way it is split across the drives - therefore making this more of an Array than a Redundant Array. If you employ RAID 0, be prepared to lose your data. You require a minimum of two hard drives for RAID 0, although having more than this is common practice for those purely interested in maximum performance.
RAID 1 is perhaps the most common RAID array - it mirrors data to multiple disks meaning that you have backups in case your main data hard drive fails. This does somewhat impact on performance, as the server will be writing to multiple drives at once. However, for many companies with mission critical applications and data, redundancy is more important than performance, making RAID 1 a common option for those looking at dedicated servers. A minimum of two hard drives are required for RAID 1, one main hard drive and another hard drive that acts as the backup drive.
RAID 10 (1+0)
RAID 10 offers the benefits of both RAID 0 and RAID 1 but without the lack of redundancy found in RAID 0 or the lack of performance found in RAID 1. This means RAID 10 offers a cocktail of performance and reliability that is perfect for the majority of companies looking at a data centre dedicated server. It does this by striping across multiple mirrored drives, meaning that you will have increased performance by striping as well as redundancy in case a hard disk fails. You will require a minimum of four hard drives for RAID 10 (2 main hard drives and 2 backup drives)
RAID 2 is similar to RAID 0 in the respect that is uses data striping, however it does this using even smaller pieces of data and also employs additional error protection that allows for corrupt data to be recovered. The issue with RAID 2 is that it is heavily equipment intensive and there are realistically cheaper options that can provide you with a similar or better service.
Similar to RAID 0 in the sense that it strips data across multiple drives but also has an additional drive to deal with error correction. As all the drives perform as one unit, you can only read or write one operation at a time, meaning RAID 3 is realistically only useful for operations dealing with single large processes/files. You'll need 3 hard drives for RAID 3 (2 main hard drives and 1 drive for error correction)
RAID 4 is similar to RAID 3 but works with large chunks of data, meaning the drives don't need to work as one unit and you can therefore run more than one read process at any one time. However, as you only have a single disk for error correction, you can still only have one write operation going ahead at any one time.
RAID 5 is commonly used in a NAS configuration (network attached storage) for file sharing and media streaming. RAID 5 stripes data as with most of the other RAID systems, however it spreads the parity across all hard drives meaning that if a hard drive fails, you won't lose the data from that disk as the parity data is elsewhere. You'll need at least three hard drives for RAID 5 although configurations generally feature at least four.
Similar to RAID 5, with an additional hard drive for parity & error correction, meaning a minimum of four hard drives are required.
You've probably already made your mind up about which RAID array makes the most sense for your business - it's important to consider performance but when it comes to the crunch, redundancy is often more important for a business. It's all very well having great performance but if you lose your mission critical data, then this will likely cost your business substantially more. Utilised correctly, RAID arrays are excellent at providing both additional performance and redundancy for your servers, meaning your data not only flows better but is also safer at the same time.