RAID - redundant array of independent disks
RAID is short for redundant array of independent (or inexpensive) disks.It is a category of disk drives that employ two or more drives in combination for fault tolerance and performance. RAID disk drives are used frequently on servers but aren't generally necessary for personal computers. RAID allows you to store the same data redundantly (in multiple paces) in a balanced way to improve overall storage performance.
The History of RAID
In 1987, David Patterson, Garth Gibson and Randy Katz published their landmark research paper, A Case for Redundant Arrays of Inexpensive Disks (RAID), which helped establish the modern RAID industry. The paper defined RAID levels 1, 2, 3, 4 and 5. Today, even more levels have been defined. More than two decades later, RAID technology remains as vibrant as ever.
Different RAID Levels
Different architectures are named RAID followed by a number and each architecture provides a different balance between performance, capacity and tolerance. There are number of different RAID levels including the following;
Level 0: Striped Disk Array without Fault Tolerance
Provides data striping(spreading out blocks of each file across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance. If one drive fails then all data in the array is lost.
Level 1: Mirroring and Duplexing
Provides disk mirroring. Level 1 provides twice the read transaction rate of single disks and the same write transaction rate as single disks.
Level 2: Error-Correcting Coding
Not a typical implementation and rarely used, Level 2 stripes data at the bit level rather than the block level.
Level 3: Bit-Interleaved Parity
Provides byte-level striping with a dedicated parity disk. Level 3, which cannot service simultaneous multiple requests, also is rarely used.
Level 4: Dedicated Parity Drive
A commonly used implementation of RAID, Level 4 provides block-level striping (like Level 0) with a parity disk. If a data disk fails, the parity data is used to create a replacement disk. A disadvantage to Level 4 is that the parity disk can create write bottlenecks.
Level 5: Block Interleaved Distributed Parity
Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance. Level 5 is one of the most popular implementations of RAID.
Level 6: Independent Data Disks with Double Parity
RAID Level 6 is similiar to RAID 5 (striped parity) except instead of one parity block per stripe there are two. With two independent parity blocks, RAID 6 can survive the loss of two disks in the group. Learn more about RAID 6 in this "Get Ready for RAID-6" article on ServrWatch.com.
Level 0+1: A Mirror of Stripes
Not one of the original RAID levels, two RAID 0 stripes are created, and a RAID 1 mirror is created over them. Used for both replicating and sharing data among disks.
Level 10: A Stripe of Mirrors
Not one of the original RAID levels, multiple RAID 1 mirrors are created, and a RAID 0 stripe is created over these.
A trademark of Storage Computer Corporation that adds caching to Levels 3 or 4.
Also called Parity RAID, EMC Corporation's proprietary striped parity RAID system used in its Symmetrix storage systems.
A RAID Standard?
Even though the RAID concept has been around since 1986 and is installed in countless millions of implementations, it is a technology that lacks standard implementations. While there is no RAID standard, at the fundamental concept level there is agreement, however RAID implementations are proprietary and unique to every vendor.
Where Can RAID Be Implemented?
There are three places to implement RAID: software, RAID controllers and storage arrays:
1. Software RAID
RAID implemented on a server by software uses internal drives or external JBOD (just a bunch of disks). The software, usually a logical volume manager, manages all of the mirroring of data or parity calculations.
2. RAID Controller
These are cards that can be added to a server and offload the overhead of RAID from the CPUs. RAID controllers are a better solution for a single server than software RAID because server CPUs spend no processing power calculation parity or managing the mirrored data. RAID controllers use either internal drives or JBOD. A server-based RAID controller can fail and be a single point of failure.
3. Storage Array
A storage array usually consists of two high-performance, redundant RAID controllers and trays of disks. All pieces of the array are redundant and built to withstand the rigors of a production environment with many servers accessing the storage at the same time. They support multiple RAID levels and different drive types and speeds. Storage arrays also usually have snapshots, volume copy and the ability to replicate from one array to another.
Top 5 Related Storage Questions
Related Webopedia References
Featured Partners Sponsored
- Increase worker productivity, enhance data security, and enjoy greater energy savings. Find out how. Download the “Ultimate Desktop Simplicity Kit” now.»
- Find out which 10 hardware additions will help you maintain excellent service and outstanding security for you and your customers. »
- Server virtualization is growing in popularity, but the technology for securing it lags. To protect your virtual network.»
- Before you implement a private cloud, find out what you need to know about automated delivery, virtual sprawl, and more. »