RAID, which stands for Redundant Array of Independent Disks, is a data storage technology that combines multiple disk drive components into a logical unit. So in short, yes, RAID is a type of storage.
What is RAID?
RAID is a way of arranging multiple hard drives to act as a single storage system for increased capacity, performance, and/or reliability. The different configurations of RAID provide various combinations of these benefits. Some key characteristics of RAID include:
- Combining multiple physical disk drives into a single logical drive
- Data is distributed across the drives in one of several ways, depending on the RAID level
- Various RAID levels provide increased data reliability through redundancy
- Implements parity checking, mirroring, or striping to achieve redundancy and/or improved performance
- Managed by RAID controllers (hardware or software)
The core benefit of RAID is that it combines multiple inexpensive drives to provide capabilities that would otherwise require more expensive hardware. By linking drives together, RAID can deliver increased storage capacities, speed, data protection, and other benefits.
Common RAID levels
There are several standardized RAID levels, each with specific characteristics in terms of performance, capacity, and fault tolerance. Some common RAID levels include:
RAID 0
RAID 0, also known as disk striping, spreads data evenly across two or more drives without parity (redundancy). The benefit is very fast data access since data can be read and written in parallel. However, it offers no fault tolerance – if one drive fails, all data is lost.
RAID 1
RAID 1, or disk mirroring, writes identical copies of data to two or more drives to provide fault tolerance. If one drive fails, data can be accessed from the other mirrored drive(s). Read performance is improved since data can be accessed in parallel. Write performance may be slower since data has to be written multiple times.
RAID 5
RAID 5 stripes data across three or more drives, along with parity data on a dedicated drive. The parity drive contains calculated error-correcting information. If a single drive fails, data can be recreated from the parity drive. RAID 5 provides good performance and fault tolerance but is more complex to implement.
RAID 6
RAID 6 is similar to RAID 5, but uses a second parity drive to protect against the failure of two disk drives. It requires a minimum of four drives and provides excellent fault tolerance and good performance for reads.
RAID 10
RAID 10 combines mirroring (RAID 1) and striping (RAID 0) for both speed and redundancy. Data is mirrored onto two drives, and those mirrored pairs are then striped across other drive pairs. RAID 10 requires at least four drives but provides fast performance along with the ability to survive multiple drive failures.
How does RAID work?
The core mechanisms that enable RAID to work are:
- Striping – Data is split into blocks which are written across multiple drives in parallel. This enables simultaneous read/write operations for faster I/O performance.
- Mirroring – Identical copies of data are maintained on duplicate drives. If one drive fails, the system switches seamlessly to the mirrored drive.
- Parity – Parity information is calculated and written to a dedicated drive. If a data drive fails, the missing data can be recreated from the parity information.
These methods are combined in various ways to create the standardized RAID levels. The specific RAID level determines exactly how data is organized and distributed across the component drives.
A RAID controller manages how the drives are organized into the RAID arrays. It coordinates the distribution of data writes and parity calculations across the drives. The controller also handles redundancy management and reforming the RAID structure if a drive fails.
Benefits of RAID
Implementing RAID provides a variety of potential benefits:
- Increased storage capacity – Combining multiple drives increases the total available storage capacity beyond the limits of a single drive.
- Improved performance – Distributing reads and writes across multiple drives allows for parallel I/O operations. Certain RAID levels also enable caching to boost performance.
- Enhanced reliability – Redundancy provided by mirroring or parity allows RAID systems to tolerate drive failures with minimal or no data loss.
- Flexibility – The ability to mix drive types and sizes allows RAID systems to be customized as needed.
- Scalability – Storage capacity can be expanded by adding additional drives.
For these reasons, RAID is commonly used for applications that require high capacity, throughput, reliability and availability. Examples include database servers, web servers, file servers, and other mission-critical systems where downtime needs to be minimized.
Drawbacks of RAID
While RAID offers important advantages, there are also some potential downsides to consider:
- Increased complexity – RAID controllers add a layer of complexity that requires management and specialized knowledge to implement and maintain.
- Single point of failure – The RAID controller itself can be a single point of failure. Sophisticated implementations may provide failover controllers.
- Lower storage efficiency – Due to parity overhead, not all capacity may be available for data storage. RAID 5, for example, requires one drive’s worth of capacity be reserved for parity.
- Slower writes – Writes may be slower due to parity calculation or data being written to multiple drives.
- Rebuilding issues – Rebuilding RAID after a drive failure impacts performance and is at risk until complete.
These potential downsides need to be weighed against the benefits when considering RAID for a storage environment.
Hardware vs. software RAID
RAID can be implemented in hardware or software:
- Hardware RAID – Uses a dedicated RAID controller, usually a PCI card installed in the server, to manage the RAID. Offloads RAID processing from the CPU.
- Software RAID – Managed by an OS-level software driver. No special hardware required. May impact CPU performance.
Hardware RAID is typically considered superior in performance due to its dedicated processing capabilities. However, software RAID provides more flexibility and may be more affordable. Many server-class systems provide hardware RAID capabilities on the motherboard.
Is RAID a backup solution?
While RAID improves availability and protects against drive failures, it is not a substitute for regular data backups. Some key reasons why:
- Does not protect against catastrophic system failure, theft, fire, etc.
- Not immune to controller failures, software bugs, human error, malware, etc.
- RAID rebuild time leaves data vulnerable until complete.
- Provides availability but not versioning – can’t roll back to previous states.
To protect against data loss scenarios outside of drive failures, RAID should be combined with regular backups stored offsite or in the cloud. Backups provide an additional line of defense against catastrophic loss.
RAID storage examples
Some examples of how RAID can be implemented for storage include:
- RAID 1 for boot drives to improve redundancy and startup speed
- RAID 5 for file servers to provide capacity, performance and protection
- RAID 10 for transactional databases to optimize I/O throughput and reliability
- Nested RAID levels combining multiple RAID configurations for added benefits
The optimal RAID implementation depends on the specific storage needs – capacity, speed, redundancy etc. Combining RAID with solid-state drives (SSDs) can further boost performance capabilities.
Evolution of RAID
RAID technologies evolved from the original RAID standard defined in the late 1980s. Some key developments include:
- RAID 2 and RAID 3 were early levels designed for performance that had limited adoption.
- RAID 4 improved parity implementation but was replaced by RAID 5 which distributed parity across drives.
- RAID 6 extended parity protection with a second parity drive.
- Nested RAID combined RAID levels for multiple benefits.
- Matrix RAID allows mixing drive sizes and RPM speeds within a single RAID group.
Software advancements, dedicated RAID processors, and new drive technologies also aided RAID improvements. RAID continues to evolve with emerging system architectures.
Alternatives to RAID
Although RAID remains prevalent, some alternatives provide overlapping benefits:
- JBOD (Just a Bunch Of Disks) – Direct access to individual drives with no aggregation or redundancy.
- ZFS – Advanced file system with built-in volume management, snapshots, and redundancy.
- Storage Spaces – Microsoft’s software-defined storage virtualization and resilience feature.
- Erasure Coding – Mathematical technique for data reconstruction similar to RAID parity.
These solutions may compete with RAID in certain use cases. However, RAID remains a proven, flexible and highly standardized model for combining multiple drives to enhance storage capabilities.
Conclusion
To summarize, RAID is undoubtedly a type of storage – it is a method for grouping drives together into redundant arrays to provide large volumes of storage with enhanced performance, capacity, and reliability. While not without some limitations, RAID delivers flexible and scalable storage solutions. It enables using inexpensive individual drives to construct high capacity drive arrays tailored to specific needs. In IT infrastructure demanding high availability and manageable storage growth, RAID remains a foundational technology.