RAID (Redundant Array of Independent Disks) is a data storage technology that combines multiple disk drive components into a logical unit to improve performance and/or reliability. RAID can boost throughput, processing power, and provide fault tolerance compared to single drives. There are several RAID levels (0, 1, 5, 6, 10 etc) that provide different combinations of performance, capacity and redundancy. Choosing the right RAID level depends on the specific needs for a storage system.
What is RAID?
RAID is a way of combining multiple physical disk drives into a single logical unit to improve capacity, performance, and reliability. The term was first coined in 1987 by researchers at the University of California, Berkeley. The key goals of RAID are to:
- Increase data transfer rates for I/O intensive applications
- Improve reliability and fault tolerance
- Increase overall storage capacity for a single logical drive
RAID achieves these goals by grouping drives together and accessing them in parallel to increase transfer speeds, and/or implementing redundancy so data can be recovered if a drive fails. A RAID setup requires a RAID controller which presents the array of drives as a single logical volume to the operating system and applications. Hardware or software RAID controllers can be used.
Benefits of RAID
Some key benefits that RAID can provide:
- Increased throughput – RAID 0 stripes data across multiple drives in parallel to improve performance for read/write intensive applications
- Fault tolerance – RAID 1 and 5 provide redundancy so data can still be accessed in the event of a single drive failure
- Improved reliability – The redundancy provided by most RAID levels reduces the risk of irrecoverable data loss if a drive fails
- More capacity – Combining multiple physical drives increases the total storage capacity beyond the limits of a single drive
By leveraging an array of disks, RAID aims to provide benefits that go beyond what can be achieved with standalone disk drives. The right RAID implementation can significantly enhance performance and resilience for critical storage needs.
RAID Levels and Configurations
There are several standardized RAID levels, each optimizing for different goals:
RAID 0
RAID 0 (also called striping) splits data evenly across two or more disks with no parity or redundancy. RAID 0 provides improved performance by spreading I/O across drives to allow parallel access. But it does not provide fault tolerance since data loss will occur if any one drive fails.
RAID 1
RAID 1 (also called mirroring) duplicates all data from one drive to a second drive to provide fault tolerance. If one drive fails, the system can instantly switch to the mirrored drive without any data loss. Performance also improves for read-heavy workloads since reads can be distributed across both drives. But the usable capacity is only equal to one physical drive.
RAID 5
RAID 5 stripes data and parity information across 3 or more disks. If any one drive fails, the missing data can be recalculated from the parity information. RAID 5 provides fault tolerance with more usable capacity compared to mirroring, but write performance may be slower than RAID 0 due to parity calculation.
RAID 6
RAID 6 is similar to RAID 5, but uses a second distributed parity scheme. This allows the array to withstand the failure of up to two disks without data loss. RAID 6 requires a minimum of 4 drives.
RAID 10
RAID 10 combines mirroring and striping by creating a striped set from mirrored drive pairs. This provides fault tolerance and improved performance but reduces overall capacity.
There are additional nested RAID levels (like RAID 50 and 60) that combine features of the basic RAID levels for specific use cases. The optimal RAID configuration depends on the goals for performance, capacity, and redundancy.
Hardware vs Software RAID
RAID can be implemented through dedicated hardware RAID controllers or via software in the operating system.
Hardware RAID
Hardware RAID uses a dedicated RAID controller card installed in the server or storage system. The controller handles all the RAID calculations and logical volume management. Hardware RAID provides performance benefits since the processing overhead is offloaded from the CPU. Battery-backed cache on the controller can further optimize write speeds. Hardware RAID requires purchasing a RAID controller card but can support any operating system.
Software RAID
Software RAID is implemented through the operating system using built-in software drivers. The CPU handles the RAID processing load. Software RAID does not require any additional hardware purchase, but at the cost of some processing overhead on the host system. Software RAID can provide an economical solution, but performance may suffer under heavy load without hardware acceleration. Software RAID is operating system dependent.
The choice between hardware and software RAID depends on budget, performance needs, and OS platform. Hardware RAID delivers better performance while software RAID is cheaper and more flexible.
RAID Performance and Capacity
The performance and capacity characteristics of a RAID configuration depend on the RAID level:
| RAID Level | Read Performance | Write Performance | Capacity Efficiency | Fault Tolerance |
|---|---|---|---|---|
| RAID 0 | Very high | Very high | 100% (ideal) | None |
| RAID 1 | High | Medium | 50% | Excellent |
| RAID 5 | High | Medium | 67% – 94% | Good |
| RAID 6 | Medium | Medium | 50% – 88% | Excellent |
| RAID 10 | Very high | High | 50% | Excellent |
- RAID 0 provides the best overall throughput, but no redundancy.
- RAID 1 and 10 excel at read speeds and fault tolerance but sacrifice capacity.
- RAID 5 and 6 offer good performance and storage efficiency along with single or double parity.
When designing a RAID array, it is important to balance performance, capacity, and redundancy to suit the specific storage needs of a system. The RAID level, number of drives, and stripe sizes all impact these characteristics.
Implementing RAID
Setting up a fully redundant RAID array involves several steps:
Select RAID Level
Choose the appropriate RAID level based on the desired balance of performance, capacity, and fault tolerance. Lower RAID levels offer capacity, while higher levels emphasize redundancy.
Choose Disks
Select compatible disks of appropriate size and speed for the array. All disks in a RAID array should typically match in terms of performance characteristics to avoid bottlenecks.
Determine Logical Volume Layout
Decide how the physical storage will be organized into logical volumes or drive groups presented to the operating system. Multiple logical RAID volumes can be created from a pool of disks.
Install RAID Controller
For hardware RAID, install the RAID controller card in a compatible PCIe slot on the server/system where the disks reside. Alternatively, verify software RAID capabilities on the OS.
Connect Disks
Connect the physical disks to the selected RAID controller ports or backplane.
Configure Virtual Disks
Use the RAID controller firmware or software tools to configure the parameters of the virtual disks, including RAID levels, stripe sizes, cache policies etc.
Initialize Storage
Perform a full initialization of the RAID array to write the configuration to disk and build parity. The operating system will now have access to the virtual disks.
Check Status
Verify successful configuration and operation of the RAID volumes by checking the controller status, running benchmark tests, and monitoring during production use.
Choosing compatible components, optimal stripe sizes, and proper RAID configuration is key for building an effective RAID array that delivers the expected performance and redundancy.
When to Use RAID
Some key situations where deploying RAID can significantly enhance a storage subsystem:
- Transactional databases – RAID 10 provides excellent performance for highly random database workloads.
- File servers – RAID 5, 6, or 10 help improve throughput for busy file servers.
- Big data analytics – Large distributed RAID arrays speed up data lakes and analytics pipelines.
- Virtualized environments – RAID helps consolidate storage into large volumes for virtual machines.
- Media editing – High speed RAID 0 arrays accelerate working with high resolution video files.
- Backup repositories – RAID 6 provides large fault tolerant volumes for backups.
Any application that demands high availability, fast I/O performance, or extra capacity is a good candidate for RAID storage. The data protection provided by RAID is also important for any critical or irreplaceable data.
Alternatives to RAID
Although RAID is a proven and popular technology, there are some alternatives that can be considered:
JBOD (Just a Bunch Of Disks)
JBOD simply combines a set of individual drives into a single volume. This allows expanding capacity but does not provide performance or redundancy benefits. JBOD is lower cost than RAID but is not suitable for mission critical data.
Erasure Coding
Erasure coding like Reed-Solomon encoding provides an alternate approach to data redundancy that requires less storage overhead compared to mirroring or parity schemes. It is gaining popularity for large-scale deployments.
Object/Cloud Storage
Object storage systems like Amazon S3 provide built-in data replication and fault tolerance without traditional RAID. Cloud storage can provide cost benefits and hands-off management.
Server SAN
Hyperconverged Server SAN solutions integrate storage into x86 servers and virtualize RAID across a cluster. This reduces hardware cost compared to dedicated SAN arrays.
All-Flash Arrays
All-flash storage systems optimize data placement across SSDs to maximize performance. They can achieve high IOPS without the complexity of large multi-disk RAID groups.
While RAID remains relevant, these alternatives provide simpler, more scalable options to consider depending on workload needs and infrastructure preferences. Hybrid approaches combining RAID and other technologies are also possible.
Conclusion
RAID delivers proven performance, resilience, and capacity improvements for storage systems by coordinating arrays of hard disk drives. Optimal RAID implementation requires matching the RAID level to the specific demands of applications and workloads. RAID improves on single disk limitations, but alternatives like cloud storage and all-flash arrays are simplifying infrastructure and storage management in some scenarios. RAID continues to provide benefits for mission critical systems that demand high availability along with fast, reliable data access. The long history and widespread adoption of RAID underline its usefulness for boosting storage capabilities across a range of computing environments.
