RAID (Redundant Array of Independent Disks) is a data storage virtualization technology that combines multiple physical disk drive components into a single logical unit for the purposes of data redundancy, performance improvement, or both.

Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the required level of redundancy and performance.

The different schemas, or data distribution layouts, are named by the word RAID followed by a number, for example RAID 0 or RAID 1.

Each schema, or a RAID level, provides a different balance among the key goals: reliability, availability, performance, and capacity.

RAID levels greater than RAID 0 provide protection against unrecoverable sector read errors, as well as against failures of whole physical drives.

RAID level descriptions:

RAID 0 consists of striping, without mirroring or parity

RAID 1 consists of data mirroring, without parity or striping

RAID 2 consists of bit-level striping with dedicated Hamming-code parity

RAID 3 consists of byte-level striping with dedicated parity

RAID 4 consists of block-level striping with dedicated parity

RAID 5 consists of block-level striping with distributed parity

RAID 6 consists of block-level striping with double distributed parity

RAID 0+1 creates a second striped set to mirror a primary striped set

RAID 1+0 creates a striped set from a series of mirrored drives

RAID 0 is used to boost a server's performance. It's also known as "disk striping." With RAID 0, data is written across multiple disks. This means the work that the computer is doing is handled by multiple disks rather than just one, increasing performance because multiple drives are reading and writing data, improving disk I/O. A minimum of two disks is required. Both software and hardware RAID support RAID 0, as do most controllers. The downside is that there is no fault tolerance. If one disk fails, then that affects the entire array and the chances for data loss or corruption increases.

With RAID 1, data is copied seamlessly and simultaneously, from one disk to another, creating a replica, or mirror. If one disk gets fried, the other can keep working. It's the simplest way to implement fault tolerance and it's relatively low cost.

RAID 5 is by far the most common RAID configuration for business servers and enterprise NAS devices. This RAID level provides better performance than mirroring as well as fault tolerance. With RAID 5, data and parity (which is additional data used for recovery) are striped across three or more disks. If a disk gets an error or starts to fail, data is recreated from this distributed data and parity block— seamlessly and automatically.

Essentially, the system is still operational even when one disk kicks the bucket and until you can replace the failed drive. Another benefit of RAID 5 is that it allows many NAS and server drives to be "hot-swappable" meaning in case a drive in the array fails, that drive can be swapped with a new drive without shutting down the server or NAS and without having to interrupt users who may be accessing the server or NAS. It's a great solution for fault tolerance because as drives fail (and they eventually will), the data can be rebuilt to new disks as failing disks are replaced. The downside to RAID 5 is the performance hit to servers that perform a lot of write operations. For example, with RAID 5 on a server that has a database that many employees access in a workday, there could be noticeable lag.

RAID 6 is also used frequently in enterprises. It's identical to RAID 5, except it's an even more robust solution because it uses one more parity block than RAID 5. You can have two disks die and still have a system be operational.

RAID 10 is a combination of RAID 1 and 0 and is often denoted as RAID 1+0. It combines the mirroring of RAID 1 with the striping of RAID 0. It's the RAID level that gives the best performance, but it is also costly, requiring twice as many disks as other RAID levels, for a minimum of four. This is the RAID level ideal for highly utilized database servers or any server that's performing many write operations. RAID 10 can be implemented as hardware or software, but the general consensus is that many of the performance advantages are lost when you use software RAID 10.

Other RAID Levels: There are other RAID levels: 2, 3, 4, 7, 0+1...but they are really variants of the main RAID configurations already mentioned, and they're used for specific cases. Here are some short descriptions of each:

RAID 2 is similar to RAID 5, but instead of disk striping using parity, striping occurs at the bit-level. RAID 2 is seldom deployed because costs to implement are usually prohibitive (a typical setup requires 10 disks) and gives poor performance with some disk I/O operations.

RAID 3 is also similar to RAID 5, except this solution requires a dedicated parity drive. RAID 3 is seldom used except in the most specialized database or processing environments, which can benefit from it.

RAID 4 is a configuration in which disk striping happens at the byte level, rather than at the bit-level as in RAID 3.

RAID 7 is a proprietary level of RAID owned by the now-defunct Storage Computer Corporation.

RAID 0+1 is often interchanged for RAID 10 (which is RAID 1+0), but the two are not same. RAID 0+1 is a mirrored array with segments that are RAID 0 arrays. It's implemented in specific infrastructures requiring high performance but not a high level of scalability.

For most small- to midsize-business purposes, RAID 0, 1, 5 and in some cases 10 suffice for good fault tolerance and performance. For most home users, RAID 5 may be overkill, but RAID 1 mirroring provides decent fault tolerance.