What is RAID?
A redundant array of independent disks, or RAID, is a way of storing the same data in different places (where the term redundant comes from) on multiple hard disks.
There are many ways to use RAID and there are several different types of array. Some use multiple disks to increase performance – think a two or three lane road instead of single, that has to serve traffic going in two directions – while others are used to increase reliability. Additionally, there is a third type of RAID that offer the best of both worlds; all of which we’ve explained below.
A RAID array appears to the operating system to be a single logical drive, regardless of how many individual drives are part of the RAID array. Both hard disks and SSDs can be used, and while it’s advised that matching drives are used in a single array for best performance, it’s not strictly required.
What kinds of RAID are there?
I know of 9 different kinds of RAID plus a performance RAID that has no redundancy built into it, however on everything outside of the high-end server environment you’ll only be offered the four main versions: RAID 0, 1, 0+1 and 5. These RAID configurations can and are used independently or in different combinations. Below are the different kinds of a RAID that are used today.
- RAID-0: This technique has striping but no redundancy of data. It offers the best performance as data is read/written to both drives simultaneously, but no fault-tolerance so if one drive fails you lose both drives data. Drives are typically added in multiples of two.
RAID-1: This type is also known as disk mirroring and consists of at least two drives that duplicate the storage of data. There is no striping. Read performance is improved since either disk can be read at the same time, meaning if one disk is busy the other is accessed. Write performance is the same as for single disk storage. RAID-1 provides the best performance and the best fault-tolerance in a multi-user system, although for a single user it’s unlikely to see as much benefit. Again drives are typically added in multiples of two.
RAID-5: This type includes a rotating parity array, thus addressing the write limitation in RAID-4. That means 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 if a single drive fails). RAID-5 requires at least three and usually five disks for the array. It’s best for systems in which performance is not so critical or which do few write operations.