RAID (Redundant Array of Independent Disks) storage is used to provide increased storage performance and reliability for computer systems. RAID combines multiple physical disk drives into one logical unit. There are different RAID levels that provide different combinations of performance, capacity, and redundancy. Some key uses cases and benefits of RAID storage include:
Performance
RAID can be used to improve performance for disk read and write operations compared to a single disk. Performance improvements come from distributing data across multiple disks that can be accessed in parallel. Some common ways RAID improves performance:
- RAID 0 stripes data across multiple disks with no redundancy. Read and write operations can be done in parallel across drives for faster I/O.
- RAID 1 mirrors disks for 100% redundancy. Read performance is improved since reads can be distributed across multiple spindles.
- RAID 5 stripes data across drives with distributed parity. The workload is distributed across drives for improved performance.
- RAID 10 combines mirroring and striping for both redundancy and parallelism. RAID 10 provides very fast read performance.
By combining multiple disk drives, RAID can provide substantial performance improvements over a single disk for transactional applications like databases or virtualized servers that require high disk I/O.
Reliability and Redundancy
RAID can provide data protection through redundancy. If a drive fails in a redundant RAID array, the missing data can be recreated from the remaining disks. Key RAID levels that provide redundancy:
- RAID 1 mirrors data between two disks. If one fails, data can be rebuilt from the other.
- RAID 5 stripes data across disks with parity information distributed across the array. The parity block can be used to recreate data if a disk fails.
- RAID 6 provides double distributed parity, so the array can survive two disk failures.
- RAID 10 mirrors stripes data for performance and can survive one disk failure per mirrored pair.
Redundant RAID protects against hardware failures and continues operating if a drive goes down. This prevents downtime and data loss for critical systems. The additional redundancy provides data protection without needing immediate replacement of failed drives.
Increased Storage Capacity
Multiple disk drives can be combined in RAID to increase the total storage capacity beyond the size of any individual drive. For example:
- RAID 0 stripes data across multiple drives for the total combined capacity.
- RAID 1 mirrors two 2 TB drives for 2 TB of usable storage.
- RAID 5 with three 2 TB drives provides 4 TB usable with distributed parity.
RAID allows scaling up available storage by using larger arrays of small commodity drives. This can be more cost effective than buying single expensive drives. Capacity can also be added incrementally as needed by expanding the array.
Hardware RAID vs. Software RAID
RAID can be implemented in hardware or software:
- Hardware RAID – Dedicated RAID controller card provides RAID processing and cache memory for the array. Takes the load off the CPU. Required for boot drives.
- Software RAID – RAID is implemented at the operating system level in software. More flexible but consumes CPU resources.
Software RAID provides more configuration options and doesn’t require specialized hardware. Hardware RAID performs better but depends on the RAID card. For performance-critical storage, hardware RAID is the best option.
RAID Levels
There are several standardized RAID levels, each optimized for a particular use case:
| RAID Level | Description |
|---|---|
| RAID 0 | Data striping across multiple disks for performance, no redundancy. |
| RAID 1 | Disk mirroring for 100% redundancy. Simple mirroring. |
| RAID 5 | Data striping with distributed parity for redundancy and write performance. |
| RAID 6 | Double distributed parity to survive up to two disk failures. |
| RAID 10 | Striping over mirrored spans for highest performance and redundancy. |
There are also nested RAID levels (like RAID 10, 50, 60) that combine striping and mirroring for specific advantages. The RAID level determines the performance profile, capacity, and redundancy characteristics.
Who Uses RAID?
Here are some examples of organizations that benefit from deploying RAID storage:
- Database servers – Use RAID 1+0 for optimal performance on transactional workloads and redundancy for uptime. Mirrored stripes prevent database bottlenecks.
- Web servers – Use RAID 10 for high availability. Can survive drive failures without downtime.
- File servers – Often use RAID 5 for large networked storage with redundancy. Parity provides an overhead over mirroring for large arrays.
- Virtual infrastructure – Combine RAID 10 for guest OS drives and RAID 5/6 for data stores. Provides performance, capacity and availability.
- Media production – Large RAID 5 arrays store massive video files while guarding against drive failures.
Any application that demands high disk performance, fault tolerance, or large amounts of storage can benefit from RAID.
Advantages of RAID Storage
Key advantages of using RAID include:
- Improved performance – By spreading I/O across drives, RAID can provide major throughput increases for read and write operations.
- Increased reliability – Redundant RAID levels (1, 5, 6, 10) provide fault tolerance and minimize downtime.
- Higher capacity – Larger storage pools by combining multiple commodity drives into one virtual disk.
- Flexibility – Many RAID levels to optimize for different use cases. Can be tuned for speed or redundancy.
For mission critical systems that demand speed, reliability, and large amounts of storage, RAID delivers significant advantages over standalone disk drives.
Disadvantages of RAID
Potential downsides to RAID include:
- Increased cost – Requires multiple drives. RAID hardware and software add expense.
- Complexity – RAID management requires additional knowledge and administrative overhead.
- Lower capacity – Redundant RAID levels (1, 5, 6) sacrifice usable space for redundancy. Not all capacity is available.
- Rebuilding issues – Reconstructing data in degraded arrays can take a long time and impact performance.
The advantages of RAID often outweigh the downsides for mission critical server storage. But RAID may not make sense for every use case, especially in smaller environments.
RAID Controllers
A RAID controller is a hardware device or software driver that manages the RAID array. Key responsibilities include:
- Reading and writing data across multiple drives in the array
- Calculating and checking parity in redundant RAID levels
- Monitoring drives for failures
- Reconstructing missing data to failed drives to rebuild the array
Hardware RAID controllers offload RAID processing from the main CPU onto dedicated controller processor and memory. This improves performance compared to software RAID using the system CPU. Hardware RAID controllers also include large cache memory to optimize disk reads and writes.
Popular Hardware RAID Controllers
- LSI MegaRAID
- Dell PERC
- HP Smart Array
- Areca ARC-series
Software RAID is implemented at the OS level, so it depends on the CPU for processing. Linux MD RAID and Windows Storage Spaces are common software RAID solutions. Software RAID provides more configuration flexibility but less performance than hardware RAID.
Choosing a RAID Level
Factors to consider when choosing the RAID level include:
- Application performance requirements – RAID 0 and 10 provide the best read/write throughput for demanding applications.
- Redundancy requirements – RAID 1, 5, and 6 offer different levels of fault tolerance.
- Available drives – More drives allow more granular choice between performance, capacity, and redundancy.
- Capacity requirements – RAID 0 and JBOD provide maximum usable capacity.
RAID 10 balances performance and redundancy for mission critical applications. RAID 5 offers a more economical choice for large storage arrays. The RAID level should align with the specific storage needs of applications and workloads.
Best Practices for RAID Setup
Guidelines for optimal RAID configuration:
- Use RAID controllers with battery-backed write caching for best performance.
- Use dedicated RAID controllers instead of motherboard SATA ports for high availability.
- Enable drive caching (write-back caching) for faster writes.
- Use hot spare drives and hot swap bays for quick rebuild after drive swaps.
- Spread drives across multiple controllers/channels for increased parallelism.
- Monitor array health to identify pre-failure warnings.
- Set email alerts for critical array events and redundancies.
Properly configuring RAID will provide faster operation and more robust redundancy for valuable data.
Migrating or Expanding Arrays
When migrating to new RAID configurations or expanding existing arrays:
- Back up data before making any changes.
- Plan drive assignments to optimize performance across channels.
- Add capacity with larger drives or incremental expansion.
- Monitor rebuild progress when expanding existing arrays.
- Consider zero-downtime migration to new array for mission critical data.
- Test expanded array before putting into production.
Careful planning and testing prevents downtime and data loss when transitioning between RAID configurations.
Alternatives to RAID
Some alternatives to RAID for storage include:
- JBOD – Just a Bunch of Disks. Allows use of standalone drives with no RAID processing overhead.
- Erasure coding – More advanced redundancy than RAID that can use different drive sizes. Used for large scale arrays.
- Object storage – Distributed storage architecture for scaling out unstructured data across clusters.
- Cloud storage – Hosted storage services like Amazon S3, Azure Storage for highly durable, scalable storage.
These alternatives can provide benefits like lower overhead, support for heterogeneous drives, and greater scalability than RAID in certain use cases.
Conclusion
RAID remains the ubiquitous standard for improving performance and reliability in server storage environments. Key takeaways about RAID include:
- Combines multiple drives into a larger, faster logical unit.
- RAID 0 stripes for pure performance, RAID 1 mirrors for redundancy.
- RAID 5 provides distributed parity for data protection in large arrays.
- RAID 10 balances high performance and redundancy for mission critical data.
- Hardware RAID controllers offload processing for better throughput.
- Choose RAID levels based on performance vs redundancy needs.
When configured with the appropriate RAID level, arrays can deliver vastly improved speed, reliability, and capacity over standalone disks for storage-intensive server workloads.