RAID (Redundant Array of Independent Disks) is a data storage technology that combines multiple disk drives into a logical unit. RAID provides increased storage performance and reliability through data redundancy.
Data redundancy means keeping multiple copies of the same data on separate disk drives. This protects against data loss if one drive fails. The redundant copies ensure the data can still be accessed from the remaining drives.
By distributing data across multiple disks, RAID can also enhance read and write speeds. There are several standard RAID configurations (RAID 0, 1, 5, 6, etc) that provide different combinations of performance, capacity and redundancy.
This article will provide an overview of common RAID levels and discuss how RAID provides data redundancy and protects against disk failures.
What is RAID?
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 takes advantage of the parallelism of multiple disks to enhance data reliability and/or performance. There are several standard RAID levels that provide different benefits:
RAID 0 combines two or more disks into a striped set, which improves performance but does not provide fault tolerance. Data is split across multiple disks, allowing concurrent disk access. However, if one drive fails, all data will be lost (1).
RAID 1 provides disk mirroring or duplexing. Data is written identically to two separate drives, providing full data redundancy. If one drive fails, data can be accessed from the other mirrored drive. However, RAID 1 doubles the hardware cost without improving performance (1).
RAID 5 stripes data and parity information across three or more disks. If one disk fails, data can be rebuilt using the parity drive. RAID 5 provides good performance and storage efficiency. However, rebuilding data after a drive failure can take a long time (1).
RAID 6 is similar to RAID 5, but uses a second independent distributed parity scheme. It can withstand the failure of two disks with no data loss. However, write speeds are slower than other RAID levels (1).
There are other RAID schemes like RAID 10 (mirrored stripes) that provide different combinations of performance, capacity and redundancy for different use cases (1).
RAID 0
RAID 0, also known as disk striping, spreads data across multiple drives without parity information. It breaks data into blocks and stripes the blocks across all the drives in the array (TechTarget). This allows for simultaneous disk access, increasing performance and providing faster data transfers.
With RAID 0, if one drive fails, all data will be lost. There is no data redundancy. RAID 0 provides performance improvements but no fault tolerance (The Plug). The benefit of RAID 0 is purely performance, ideal for non-critical data where speed is most important. The more drives added, the larger the performance gain. But the risk of data loss also rises with more disks.
RAID 1
RAID 1, also known as disk mirroring or RAID mirroring, is a RAID configuration that utilizes two identical copies of data written to two separate drives (TechTarget, n.d.). The drives contain duplicate copies of the data, acting as mirrors of one another. If one drive fails, the system can instantly switch to the other drive without any interruption to data availability. This provides fault tolerance and protects against data loss in the event of a drive failure (Enterprise Storage Forum, 2021).
With RAID 1, all data is written to both drives simultaneously in a parallel fashion. When data is read, it can be accessed from either of the mirrored drives. RAID 1 provides redundancy and increases fault tolerance, though it doubles the cost since twice the number of drives are needed compared to a single drive. RAID 1 is commonly used in mission critical storage systems where high availability and data protection are crucial (Enterprise Storage Forum, 2021).
RAID 5
RAID 5 is a storage configuration that uses distributed parity to provide redundancy and fault tolerance. With RAID 5, data and parity information are striped across three or more drives. The parity information is distributed evenly across all the drives and provides the ability to reconstruct data if one of the drives fails.
Here’s how distributed parity works in RAID 5:
- Data is split into blocks and striped across all the drives in the array.
- Parity information is calculated and written across the drives as well. Each drive contains a parity block corresponding to the data blocks in the same stripe.
- The parity blocks are distributed evenly among the drives and not concentrated on any single drive.
- If any single drive fails, the parity blocks on the surviving drives can be used to reconstruct the data that was on the failed drive.
- This provides fault tolerance and allows the RAID 5 array to continue operating with full data redundancy in the event of a single drive failure.
The distributed nature of the parity in RAID 5 provides redundancy without sacrificing too much usable capacity. RAID 5 requires a minimum of three drives and is a popular choice for data protection in storage systems.
RAID 6
RAID 6 utilizes double distributed parity to provide fault tolerance in the event of up to two drive failures. This means there are two separate parity data blocks distributed across the array’s drives (TechTarget). The parity information is calculated using an XOR operation and spread across different drives. If one drive fails, the missing data can be recreated from the remaining data and parity drives. If a second drive fails, the second parity block allows recovery as well. The dual parity provides an extra layer of redundancy compared to RAID 5, which has a single parity block (Igor Ostrovsky).
Is RAID redundancy?
Redundancy in RAID refers to data protection through the duplication of critical components in a RAID array. If a drive fails in a redundant RAID array, the data on the failed drive can be reconstructed from the remaining drives. Not all RAID levels provide redundancy. The most common redundant RAID levels are:
RAID 1: Disk mirroring provides 100% redundancy by duplicating all data across two or more drives. If one drive fails, the data remains fully intact and accessible on the other mirrored drive(s) (Source).
RAID 5: Block-level striping with distributed parity provides redundancy by using parity information distributed across all drives. If one drive fails, its data can be reconstructed from the parity information (Source).
RAID 6: Similar to RAID 5 but with double distributed parity. Can withstand the loss of up to two drives. Provides very high redundancy (Source).
In contrast, RAID 0 provides no redundancy at all. If a single drive fails in a RAID 0 array, all data in the array will be lost. RAID 10 provides redundancy through mirroring but does not have distributed parity like RAID 5/6.
Benefits of redundancy
One major advantage of data redundancy is improved reliability and fault tolerance. If a drive in a RAID array fails, the data still remains accessible on the other drives (Source). The redundant drives provide a backup copy of the data, ensuring continued access even if one component fails. This gives RAID far greater fault tolerance compared to a single drive.
By duplicating critical data across multiple disks, RAID can withstand disk failures and continue operating with minimal or no downtime. The redundant disks help mask and absorb errors, acting as a safety net. If one disk becomes corrupted or inaccessible, the data remains intact and online due to the redundant copies. This provides high availability and business continuity.
In addition, performance remains unaffected if a disk fails, as the input/output workload is distributed across the remaining disks. The redundant nature of RAID delivers failover protection without compromising speed or uptime. Overall, redundancy dramatically improves the reliability and resilience of storage.
Downsides of redundancy
While data redundancy has its benefits, there are also some downsides to consider. The main downsides are increased costs and complexity.
Maintaining redundant copies of data requires additional storage capacity, which increases costs for hardware and infrastructure. There is also added complexity in keeping redundant copies synchronized through replication and other mechanisms. This requires additional software and administrative overhead.
According to the Inslycle blog article “Types of Customer Data Redundancy,” managing redundant data across systems or record types introduces complexity and can be challenging: https://blog.insycle.com/types-of-customer-data-redundancy
Redundancy may also complicate applications and analytics, as some systems may need to consolidate or normalize data from redundant sources. There can also be inconsistencies if redundant data gets out of sync.
In summary, the main downsides of redundancy are increased costs for storage and management, as well as added complexity in systems and operations.
Conclusion
RAID is a storage technology that combines multiple hard drives into a single storage system to increase performance, capacity, or reliability. While there are multiple RAID configurations, the most common RAID levels used today for redundancy are RAID 1, RAID 5, and RAID 6.
RAID 1 provides redundancy by mirroring data across two drives. If one drive fails, the data remains intact on the other mirrored drive. RAID 5 stripes data and parity information across three or more drives, allowing for one drive failure without data loss. RAID 6 is similar but can withstand up to two drive failures.
The main benefit of building redundancy into a RAID configuration is protection against hardware failures and improved uptime. If a drive fails, the RAID system can continue operating using the remaining drives. However, redundancy comes at the cost of usable capacity, as drives are required for the redundant data. RAID also increases complexity and can impact performance.
In summary, RAID can provide different levels of redundancy depending on the RAID level used. For many applications where uptime and data protection are critical, the benefits of RAID redundancy outweigh the capacity tradeoff. But redundancy adds cost and complexity, so it may not make sense in all scenarios.
