RAID stands for Redundant Array of Independent Disks. It is a data storage technology that combines multiple disk drive components into a logical unit. RAID allows data to be distributed across multiple disks, while also providing data redundancy in case of drive failure. Some key benefits of RAID include increased storage capacity, faster data access, and fault tolerance.
What is RAID?
RAID is an acronym that stands for Redundant Array of Independent Disks. It is a data storage technology that interconnects multiple physical disk drives and presents them as a single logical storage unit to the operating system. The main goals of RAID are to increase data reliability and/or increase input/output performance. RAID achieves this through data redundancy, where data is copied to multiple disks, and parallelism, where data requests can be processed concurrently by multiple disks.
The term “array” in RAID refers to the collection of multiple independent disks that are arranged together. The “redundant” part of the acronym refers to storing duplicate copies of data in the array so that data can still be accessed in the event of a disk failure. Overall, RAID combines smaller, less reliable disks into a larger and more reliable logical storage unit through redundancy and data distribution techniques.
Key Characteristics of RAID
- Combines multiple physical disks into a single logical unit
- Data is distributed across multiple disks (striping)
- Redundant data copies provide fault tolerance
- Parallelism allows concurrent data requests
- Hardware or software implementation
Brief History of RAID
The initial concept of RAID was introduced in 1987 by researchers David Patterson, Randy Katz, and Garth Gibson at the University of California, Berkeley. Their seminal paper, “A Case for Redundant Arrays of Inexpensive Disks (RAID)”, outlined the fundamental RAID concepts that are still used today.
The researchers recognized the disconnect between the rapidly improving processing power of computers and the slower mechanical performance advances of hard disk drives. Their solution was to combine multiple inexpensive disks together into an array that could deliver faster data transfer rates and greater fault tolerance relative to single large drives.
The original RAID paper defined five different RAID levels from RAID-1 to RAID-5, each providing various tradeoffs between features like speed, data redundancy, and cost. Over time, additional RAID levels were defined by industry vendors, expanding on the initial research concepts.
In the late 80s and early 90s, hardware RAID controllers emerged to implement RAID systems. Initially adopted on servers and highend workstations, RAID became more affordable and accessible for mainstream desktop PCs during the 1990s. Today RAID is a ubiquitous data storage technology, available as dedicated hardware, host bus adapter cards, or integrated software solutions.
How Does RAID Work?
At the core of RAID is the distribution, or “striping”, of data across multiple disk drives in the array. This distributed storage approach is what enables the key performance and reliability benefits of RAID.
There are several techniques that RAID uses to achieve these benefits:
Striping
RAID stripes data across multiple disks. Sequential data blocks are divided up and written in rounds across the disks in the array. For example, Block 1 goes to Disk 1, Block 2 to Disk 2, Block 3 to Disk 3, and so on until all disks have received one block. The next set of blocks goes to the next drive in order.
This striping distributes the data load evenly across disks, allowing concurrent read/write operations on multiple drives for improved performance.
Mirroring (RAID 1)
RAID 1 duplicates all data on a second drive to provide fault tolerance. Writes must go to both drives, while reads can be handled independently by either drive. If one drive fails, the system can instantly switch to the mirrored drive without any data loss.
Parity (RAID 5)
RAID 5 distributes both data and parity information across all the drives. Parity is the calculation of error correcting information used to reconstruct data if a drive fails. Having the parity distributed avoids data bottlenecks. If a drive fails, its data can be recreated from the data and parity on the other remaining drives.
Spanned Sets (RAID 0)
RAID 0 combines two or more drives into a larger single logical volume, with data striped across both drives for performance, but without any redundancy. The storage capacity is the total of all drives, while read/write speeds can approach the sum of the individual drives.
RAID Levels Explained
There are several standardized RAID levels, each designed with specific performance, redundancy, and cost tradeoffs in mind. Some key RAID levels include:
RAID 0
Goal – Increased performance
How it works – Striping distributes data across multiple drives, but without redundancy between drives.
Benefits – Fast performance, simple implementation. Combined storage capacity equals sum of all drives.
Drawbacks – No fault tolerance. If one drive fails, all data is lost.
RAID 1
Goal – Increased redundancy
How it works – Disk mirroring provides an exact copy of data on a second drive.
Benefits – Very high read performance and fault tolerance. Instant failover if a drive goes down.
Drawbacks – High cost since it doubles the required number of hard disks.
RAID 5
Goal – Balance between performance, redundancy, and efficiency
How it works – Data is striped across drives with distributed parity information.
Benefits – Very good read performance and fault tolerance. Can survive a single drive failure. Efficient use of storage capacity.
Drawbacks – Parity calculations can impact write performance.
RAID 6
Goal – High fault tolerance with minimal performance impact
How it works – Like RAID 5 but with double distributed parity. Can handle up to two drive failures.
Benefits – Excellent read performance. Can survive multiple drive failures with no data loss.
Drawbacks – Higher complexity than other RAID levels.
There are additional RAID levels available that provide more niche capabilities, but RAID 0, 1, 5, and 6 are among the most common implementations.
Advantages and Disadvantages of RAID
Some key advantages of using RAID include:
- Increased storage capacity – Combining multiple drives expands the total available storage space compared to a single large drive.
- Faster data access – Distributing data across multiple disks allows for concurrent operations.
- Fault tolerance – Redundant data ensures access to data even with disk failures.
- No single point of failure – RAID remains operational even if some component fails.
Potential disadvantages or limitations of RAID can include:
- Increased complexity – RAID controllers add hardware and software complexity.
- Extra computation – Parity calculations can consume processing overhead.
- Rebuilding issues – Reconstructing data after a failure can impact performance.
- Cost – Additional or more expensive disks are required compared to single disks.
Overall, RAID delivers important data storage reliability and performance enhancements for many workloads. But the added complexity should be weighed against needs when considering RAID solutions.
Software vs Hardware RAID
RAID can be implemented through dedicated hardware RAID controllers or via software-based RAID.
Hardware RAID
Hardware RAID uses a specialized RAID controller card for processing and distributing data across drives. The controller handles all RAID logic and presents the array as a single drive to the operating system. Hardware RAID provides excellent performance since it uses dedicated processing resources. But the controller card adds cost and represents a single point of failure.
Software RAID
Software RAID relies on processing by the system’s main CPU and OS drivers. It can be implemented on commodity computers without any special hardware. Software RAID provides more flexibility in terms of management and drive selection. But performance can suffer under heavy workloads since it competes with other system resources for CPU cycles.
Software RAID now includes many enterprise-grade resiliency features and is common even in data centers today. For home or small offices, software RAID provides a low cost implementation leveraging modern multicore processors.
RAID Implementation Considerations
Some key factors to consider when implementing RAID include:
- Required redundancy level – How much fault tolerance and drive failure protection is needed?
- Performance requirements – Will faster data access or I/O operations improve workflow?
- Available drives – Existing drives in a system may dictate some RAID options.
- Cost – RAID has higher hardware costs than single disks.
- Complexity – Software RAID avoids added hardware complexity.
It’s also important to select the appropriate RAID level to match the specific use case. Performance-oriented applications may leverage RAID 0, while mission critical data requires fault tolerant RAID 6. Consulting with IT infrastructure specialists can help determine optimal RAID configurations.
Who Uses RAID and Why?
RAID is used in a wide variety of computing environments where performance, reliability, and large storage capacities are important considerations, including:
- Data centers – Critical for ensuring uptime and preventing data loss on enterprise servers.
- Web servers – Provides performance for high traffic apps and websites.
- File servers – Shared storage and access demands make RAID essential.
- Database servers – Fast I/O needed for high transaction databases and data warehousing.
- Workstations – Power users rely on RAID for editing media, analyzing data, and productivity.
- Gaming PCs – Gamers use RAID 0 for fast access to game installs and load times.
Essentially any computer system that demands increased storage, speed, reliability can benefit from the capabilities of RAID. The technology protects valuable data and delivers vital performance for a wide range of applications and users.
The Future of RAID
RAID technology will continue evolving to meet growing storage demands and take advantage of emerging innovations like:
- New interfaces – Adoption of faster drive interfaces like PCIe NVMe to boost performance.
- Integrated RAID – Chipsets and motherboards with built-in RAID capabilities.
- Advanced file systems – File systems designed specifically for RAID-based fault tolerance and error correction.
- Auto-tuning – RAID controllers that self-monitor and auto-configure for optimal performance.
- AI integration – Machine learning to model drive behavior and predict failures.
While the fundamentals remain unchanged, we will see continual enhancements to management interfaces, drive integration, and back-end RAID processing. This will enable RAID systems to adapt to evolving storage trends like solid state drives, shared storage, and flash arrays.
Conclusion
RAID delivers an essential mix of performance, redundancy, and expansive storage capacity. By combining multiple inexpensive disks, RAID can meet demanding storage requirements at large and small scales. Although not always necessary for basic storage needs, for mission critical data or high performance applications, RAID remains a foundational technology even into the future.
