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. Data is distributed across the drives in one of several ways called RAID levels, depending on the required level of redundancy and performance.
The main reasons to use RAID are to increase data reliability and/or increase input/output performance. It provides data redundancy by replicating data across multiple disks, ensuring continuous availability in case of disk failure. Different RAID levels use different data distribution layouts across the disks, optimizing for various use cases.
Why is RAID used?
There are several key benefits of using RAID:
Increased Reliability and Fault Tolerance
Disk drives have relatively high failure rates compared to other computer components. By replicating data across multiple drives in RAID configurations, the array can remain operational even if one or more disk drives fail. Failed disks can be replaced or recovered without downtime or data loss.
Improved Performance
RAID can improve input/output operations per second (IOPS) by distributing data across multiple disks that can operate in parallel. For read operations, multiple disks can be read simultaneously. For writes, data can be written to multiple disks concurrently.
Capacity Scaling
RAID allows easy scaling of storage capacity by adding more disk drives to the array. Large RAID arrays can be created to build petabyte-scale storage systems.
Economy
Larger capacity disk drives are more economically priced. Building a high-capacity storage system with RAID using fewer large drives can be more cost effective than many smaller disks.
What are the Different RAID Levels?
There are several standard RAID levels, each optimized for different use cases:
RAID 0
RAID 0 stripes data across the disks for performance, but does not provide redundancy. It improves performance by spreading reads and writes across multiple disks that operate in parallel. However, if any disk fails, all data in the array will be lost.
RAID 1
RAID 1 duplicates (mirrors) data across disks for redundancy. Any data written to one disk is also written to the other mirrored disks. If one disk fails, data remains available from the other disks. Performance is slower than RAID 0 due to the write overhead of duplicating data.
RAID 5
RAID 5 stripes data across disks like RAID 0, but also generates and stores parity information that can be used to reconstruct data if a disk fails. It requires a minimum of 3 disks. RAID 5 offers a balance of speed, capacity, and redundancy for typical use cases.
RAID 6
RAID 6 is similar to RAID 5, but uses an additional parity block for redundancy. It can withstand the failure of up to two disks. With the extra parity, write performance is slower than RAID 5, but allows recovery from two disk failures.
RAID 10
RAID 10 combines mirroring (RAID 1) and striping (RAID 0) for both speed and redundancy. Data is mirrored onto pairs of disks, and those mirrored pairs are then striped across multiple disks. RAID 10 requires a minimum of 4 disks.
RAID System Architectures
There are two basic architectures used to implement RAID systems:
Hardware RAID
A hardware RAID controller manages the RAID system, performing the parity calculations and distributing IO without using host system resources. Hardware RAID provides better performance but costs more than software RAID.
Software RAID
A software RAID system uses the main system CPU and OS disk drivers to perform the RAID algorithms and manage the array. Software RAID has minimal hardware costs, but uses CPU resources and can impact performance.
Choosing the Right RAID Level
Factors to consider when choosing a RAID level include:
– **Performance** – IOPS capabilities for random vs sequential workloads
– **Redundancy** – Number of disk failures that can be tolerated
– **Capacity** – Total usable array capacity
– **Cost** – Controller vs software RAID implementation
RAID 0 maximizes performance while RAID 1 optimizes redundancy. RAID 5 provides a good balance with reasonable cost. For mission critical data, RAID 6 or 10 provide the safest redundancy. The RAID level choice depends on the specific storage needs of the application and acceptable tradeoffs.
Should RAID be Used for Modern Storage?
While RAID remains in widespread use, some modern storage technologies are changing the conversation:
Advances in Disk Reliability
Individual disk drive reliability continues to improve, reducing the chance of failure. High quality enterprise SATA or SAS drives can have sufficiently low annual failure rates that RAID redundancy becomes less critical.
Emergence of SSDs
Solid state drives (SSDs) have no moving parts and better reliability than traditional spinning hard drives. The redundancy of RAID becomes less important with SSD storage. However, RAID can still be useful to increase performance.
Cloud and Software-Defined Storage
Modern storage solutions like distributed filesystems, object stores, and cloud services provide redundancy across servers or data centers. They reduce reliance on RAID at the individual server level.
Erasure Coding
Erasure coding can match RAID 5’s redundancy with lower overhead. It is used in some modern software storage systems as an alternative to traditional RAID.
Conclusion
While emerging technologies are reducing reliance on traditional RAID, it still powers much of the world’s storage infrastructure due to its simplicity, performance, and ability to tolerate disk failures. RAID delivers configurable tradeoffs among speed, capacity, and reliability. Companies continue using RAID systems alongside new storage technologies chosen based on their specific needs. RAID remains a versatile, economical, and effective solution for achieving redundancy and high availability with commodity disk drives.