RAID (Redundant Array of Independent Disks) drives are used to increase performance, capacity, and reliability of data storage. RAID drives use multiple physical hard drives and organize them together into one logical drive for improved performance or redundancy.
What is RAID?
RAID is a technology that combines multiple disk drive components into a logical unit. Data is distributed across the drives in one of several ways called “RAID levels”, depending on what level of redundancy and performance is required. RAID drive configurations are used to protect data against drive failures and improve performance for I/O-intensive applications.
The main goals of RAID drives are:
- Improve reliability and fault tolerance
- Increase I/O performance
- Increase storage capacity using multiple drives
RAID drives achieve this by grouping drives together and accessing them in parallel to increase transfer speeds and by introducing redundancy so data can be recovered if a drive fails.
Brief history of RAID
The term “RAID” was first defined in a 1987 paper titled “A Case for Redundant Arrays of Inexpensive Disks (RAID)” by researchers at the University of California, Berkeley. This paper discussed how an array of inexpensive drives could replace large expensive drives and provide better performance, reliability, and lower cost. Key benefits outlined included:
- Data redundancy to protect against drive failures
- Improved read/write performance by distributing I/O across drives
- Capacity expansion by combining multiple drives
Since then, RAID technology has evolved with new RAID levels and drive interfaces, but the core goals remain the same. RAID is now considered an essential technology for storage systems from desktops to enterprise data centers.
How does RAID work?
RAID combines multiple physical drives into a single logical storage unit. Data is distributed across the drives according to the RAID level. The RAID level determines how data is organized and how redundancy is implemented. The main components of a RAID drive system are:
- RAID controller: This is a hardware or software controller that organizes the drives and handles the distribution of data across the array.
- RAID drives: These are the individual hard disk drives that make up the array. Commercial RAID systems typically use specially engineered drives designed for RAID environments.
- RAID software: The RAID controller uses proprietary software or drivers to organize the drives according to the RAID level and virtualize them into one logical drive.
The RAID controller stripes and mirrors data across the drives automatically according to the RAID level. If a drive fails, the controller manages the rebuild process. The RAID appears to the computer as a single storage unit. Advanced RAID controllers also include caching to improve performance.
Benefits of using RAID drives
There are several key benefits to using RAID drives:
- Increased storage capacity – RAID allows combining multiple cheaper lower capacity drives to get a larger storage volume.
- Faster performance – By striping data across multiple drives, RAID can increase read and write speeds, especially for sequential operations.
- Redundancy and fault tolerance – RAID provides protection against drive failure with parity or mirroring of data across drives.
- Improved reliability – The redundancy provided by RAID results in more reliable storage than a single drive.
- Flexibility – Different RAID levels allow optimization for performance, redundancy, and capacity as needed.
Overall, RAID delivers more speed, capacity, and reliability than standalone disk drives at a lower cost by combining inexpensive drives. Critical business systems and applications can benefit greatly from deploying storage in a RAID configuration.
Disadvantages of RAID
Some potential downsides to RAID include:
- Increased complexity for setup and management
- Potential for lower performance on random write operations depending on RAID level
- Higher cost than single drives for redundancy features
- Requires RAID controller and may have software licensing costs
- RAID rebuild times can be very long for large arrays if a drive fails
- The entire array is at risk during rebuilds if additional drive failure occurs
Types of RAID (RAID levels)
There are several standardized RAID levels, each designed for different use cases:
RAID 0
- Also called disk striping
- Data is split across drives in stripes
- No redundancy – all data lost if any one drive fails
- Fastest performance, combines capacities of all drives
- Used where speed is critical and redundancy is less important
RAID 1
- Disk mirroring
- Duplicates data across mirror drives
- Provides redundancy, can survive one drive failure
- Slower write performance than RAID 0 due to mirroring overhead
- Used where redundancy is critical
RAID 5
- Block-level striping with distributed parity
- Parity allows recovery from one drive failure
- Good balance of speed, capacity, and redundancy
- Widely used in servers
RAID 6
- Block-level striping with double distributed parity
- Can survive loss of two drives
- Used where redundancy is critical, but RAID 1 would be too costly
RAID 10
- Combines mirroring and striping (RAID 1 + RAID 0)
- Mirrors stripes data across drives
- Provides redundancy of RAID 1 plus speed of RAID 0
- Can survive multiple drive failures if failures are on different mirrors
- Used for mission critical systems needing speed and redundancy
There are additional nested and non-standard RAID levels available as well, providing a wide range of implementation options to balance performance, capacity, and redundancy as needed.
Hardware vs. software RAID
RAID can be implemented through dedicated hardware RAID controllers or via software RAID:
- Hardware RAID – Uses a dedicated RAID controller, usually in form of a plug-in card. Manages all RAID functions via dedicated hardware. Provides best performance but at increased cost.
- Software RAID – Managed by OS and drivers. Cheaper to implement but puts load on system CPU. Performance depends on CPU power.
Hardware RAID is generally preferred for mission critical environments that require the best performance. Software RAID provides a lower cost option where CPU resources are adequate.
Choosing the optimal RAID level
Factors to consider when choosing RAID level:
- Required redundancy and fault tolerance
- Capacity requirements
- Random vs sequential workloads
- Read vs write performance needs
- Number of drives available
- Cost, performance, and capacity tradeoffs
Typically RAID 5 provides a good balance for general purpose use with good redundancy and performance. RAID 10 provides maximum performance and redundancy for critical applications but at increased cost. The RAID level choice depends on the specific requirements.
Typical uses cases for RAID drives
Some common use cases for RAID drives include:
- File and application servers – Used for improved performance, capacity, and redundancy. RAID 5 or RAID 10 typical for high demand environments.
- Network attached storage (NAS) – Home and business NAS devices use RAID to provide large capacities combined with redundancy.
- Transactional databases – Databases use RAID to achieve fast data access along with protection against drive failure.
- Virtualization and cloud storage – RAID used in backend to provide capacity, speed, and reliability. Allows uninterrupted operations.
- High performance workstations – Video editing, engineering, scientific use RAID 0 for maximum speed.
- Media streaming and surveillance – Requires high capacity redundant storage with steady performance. Fully populated low-end RAID 5/6 typical.
Any application that demands speed, large capacity, and reliability can benefit from a properly designed RAID storage system.
Implementing a RAID system
Steps to implement a RAID system:
- Assess application requirements – capacity, speed, redundancy needed, etc.
- Select RAID level based on requirements and tradeoffs
- Choose hardware vs software RAID
- Select compatible RAID controller and drives
- Determine number of drives needed based on capacity and RAID level
- Install RAID controller and drives into server/computer
- Configure RAID settings on controller to desired RAID level
- Initialize RAID array – controller will build parity and/or mirrors
- Format RAID array with desired file system (ex: NTFS, ext4)
Additional steps like partitioning, creating volumes, tuning, and ongoing monitoring may be required. Follow best practices for RAID configuration and maintenance.
Best practices for RAID setup and maintenance
Some best practices for optimal RAID performance, reliability, and maintenance:
- Use quality enterprise-grade RAID controllers and disk drives designed for RAID
- RAID 1+0 provides excellent performance/redundancy but requires even drive counts
- Keep firmware on RAID controller and drives updated
- Enable drive failure alerts and monitoring
- Hot spare drives allow automatic rebuild if a drive fails
- Stagger drive replacements to avoid multiple failures
- Scrub RAID array weekly or monthly to proactively detect and repair errors
- Monitor rebuild times as prolonged rebuilds indicate issues
- Schedule regular backup of RAID array to guard against catastrophic failure
Careful RAID setup, monitoring, and maintenance helps optimize performance while avoiding downtime.
Advantages of hardware vs. software RAID
| Hardware RAID | Software RAID |
|---|---|
|
|
Hardware RAID provides better performance and reliability while software RAID costs less. Virtualized environments have mostly shifted towards software RAID due to flexibility. Hardware RAID still preferred for mission critical systems.
Disadvantages of popular RAID levels
| RAID Level | Disadvantages |
|---|---|
| RAID 0 | No fault tolerance, total data loss with single drive failure |
| RAID 1 | 50% storage efficiency due to mirrors |
| RAID 5 | Poor random write performance, high rebuild times |
| RAID 6 | High rebuild times, poor random write performance |
| RAID 10 | Up to 50% storage efficiency due to mirroring |
Each RAID level involves tradeoffs between performance, capacity, redundancy, and cost. The disadvantages must be weighed against the benefits for the intended use case.
When to choose hardware vs. software RAID
Hardware RAID is preferred when:
- Maximum performance and reliability are critical
- Booting from the RAID array is required
- High capacity array is needed beyond software RAID limits
- Hardware acceleration is beneficial for large video/image files
Software RAID may be suitable when:
- Cost saving is priority over maximum performance
- Ease of management through OS tools is beneficial
- Array capacity is within software RAID limits
- Virtualized environments where RAID is managed by hypervisor
For mission critical systems, databases, enterprise applications, and high capacity arrays, dedicated hardware RAID still provides significant advantages. Software RAID is often used in budget systems, smaller arrays, or virtual environments.
Conclusion
RAID technology delivers improved performance, fault tolerance, and capacity by combining multiple drives. Choosing the optimal RAID level and proper configuration allows building high capacity storage arrays tailored to application needs at lower costs than large single drives. With proper setup and maintenance, RAID can provide fast, resilient storage well-suited for mission critical business systems and applications.