RAID 5 is a redundant array of independent disks (RAID) configuration that stripes blocks of data and parity information across multiple disks. RAID 5 was first introduced in the late 1980s as a cost-effective solution for improving storage reliability and performance. Its key benefit is the ability to withstand a single disk failure without losing data or access to the array.
RAID 5 works by distributing data and parity information in stripes across all the disks in the array. The parity information allows the data from a failed disk to be recreated from the remaining disks. This eliminates the need for mirroring found in RAID 1. Compared to RAID 0 which offers no redundancy, RAID 5 provides fault tolerance and protection against data loss.[1]
By avoiding the cost of full duplication found in RAID 1, RAID 5 provides efficient use of storage capacity. It requires a minimum of three disks, with additional disks improving performance. RAID 5 became a popular choice for cost-effective storage arrays in the 1990s and 2000s.[2]
How RAID 5 Works
RAID 5 utilizes distributed parity and striping to provide redundancy and improved performance. Unlike RAID 1 which uses mirroring, RAID 5 spreads parity information across all the drives in the array (Source). This distributed parity allows the array to sustain a single disk failure without losing data. If a disk fails, the missing data can be recreated from the remaining data and parity information.
RAID 5 offers excellent read performance since data is striped across multiple disks, allowing segments of a file to be read in parallel. However, write performance suffers due to the parity calculation on each write. When new data is written, the parity must be updated across the array, requiring additional disk accesses. This write penalty gets worse as more disks are added. Rebuild times for a failed drive are also lengthy since the parity information is used to reconstruct the lost data (Source).
RAID 5 Drawbacks
RAID 5 has two major drawbacks related to disk failures:
First, RAID 5 can have very long rebuild times when replacing a failed disk. Since the array has to recalculate parity information across all disks when rebuilding, the larger the disks, the longer the rebuild takes. As this source notes, these long rebuild times increase the risk of data loss if additional disks fail before the rebuild completes.
Second, RAID 5 is vulnerable to multi-disk failures. If two disks fail simultaneously before a previous disk is rebuilt, the entire RAID 5 array will be lost. As disk sizes continue to increase, the probability of multi-disk failures also increases during long rebuild times. Again, this leads to heightened risk of irrecoverable data loss.
Due to these factors, some experts argue that RAID 5 is no longer a reliable option, especially for arrays with large high-capacity disks.
Alternatives to RAID 5
There are several alternatives to RAID 5 that offer greater redundancy and can better handle multiple disk failures:
RAID 6
RAID 6 is considered the successor to RAID 5, as it offers double distributed parity like RAID 5 but can withstand the failure of two disks simultaneously [1]. This added fault tolerance comes at the expense of reduced usable capacity since a second drive’s worth of space is devoted to parity. However, the ability to survive two failed disks makes RAID 6 better suited for large arrays or mission critical data where uptime is paramount.
RAID 10
RAID 10 provides redundancy through data mirroring while also increasing performance via data striping. By making an exact copy of all data across paired disks, RAID 10 can survive multiple simultaneous drive failures as long as each failure occurs on a different mirrored set. The tradeoff is having only 50% usable capacity since half the array is devoted to mirroring. RAID 10 is faster for both reads and writes compared to RAID 5, making it more suitable for heavy I/O workloads.[2]
RAID 50/60
RAID 50/60 combines RAID 5 and RAID 0/RAID 6 and RAID 0 features to provide a balance of performance, capacity and redundancy. RAID 50 stripes data across multiple RAID 5 sets while RAID 60 utilizes RAID 6 sets instead. This allows RAID 50/60 to handle multiple disk failures across different drive subsets. Performance is also boosted from the RAID 0 striping of the RAID 5/6 sets. RAID 50/60 require a minimum of 6 disks and are common options for larger arrays.
When to Use RAID 5
RAID 5 can still be a good option for certain use cases where cost savings and moderate fault tolerance are priorities over maximum performance and redundancy. Some situations where RAID 5 may make sense include:
For budget arrays. RAID 5 only requires one parity drive, versus two for RAID 6. This can help lower the cost of an array. The trade-off is less redundancy than RAID 6.
For non-critical data. Since RAID 5 has a higher risk of data loss if multiple drives fail, it should only be used for data that is non-essential or can be recreated if lost. Things like media files, backups, etc. Critical business or personal data is better suited to RAID 6 or 10.
When used with smaller drive sizes, like 2TB or less. The rebuild times are more manageable with smaller drives if a failure occurs.
For sequential/streaming read workloads. RAID 5 performs well for workloads like media streaming that are mostly sequential reads.
As a stepping stone from RAID 0. RAID 5 can provide a minimum level of fault tolerance coming from RAID 0.
When cost savings outweigh performance needs. The lower cost of RAID 5 may make sense if budget is the primary concern.
When to Avoid RAID 5
RAID 5 is not recommended for certain use cases where its drawbacks can cause major issues. Two key scenarios where experts advise avoiding RAID 5 are:
Mission Critical Applications
RAID 5 should be avoided for mission critical applications that cannot afford any downtime. The rebuild times on a degraded RAID 5 array with large drives can be extremely long. If another disk fails during this rebuild, the entire array will be lost. The consequences of data loss or downtime are too high for critical applications.
High Disk Count Arrays
As the disk count in a RAID 5 array increases, the probability of a second disk failing during a rebuild also increases. With higher capacity disks, rebuilds also take longer. For these reasons, RAID 5 is not recommended for arrays with 6 or more disks. The risk of a second disk failure is too high.[1]
Instead of RAID 5, RAID 6 or RAID 10 are better options for mission critical and high disk count arrays, despite increased cost. The redundancy of RAID 6 or 10 makes them more fault tolerant.
Best Practices
When using RAID 5, there are some best practices to follow for performance and reliability:
Schedule regular scrubs to check data integrity. Scrubs read all the data blocks and checksum them to verify there are no errors. This can reveal failed disks before they lead to data loss. Scrubs should be scheduled monthly or quarterly depending on the storage workload. (Best Practices for Configuring the IBM System Storage DS3300 and an IP SAN)
Use hot spares to allow automatic rebuilding if a disk fails. The hot spare immediately begins rebuilding the RAID set on the new disk. This reduces the window of vulnerability to a second disk failure. (Best Practices for Configuring the IBM System Storage DS3300 and an IP SAN)
Monitor disks with SMART to detect signs of failure before it happens. Most drives support Self-Monitoring, Analysis and Reporting Technology (SMART) to monitor internal attributes like reallocated sectors. Tracking these over time reveals degrading disks. (Thermal control criteria)
The Future of RAID 5
RAID 5 faces challenges going forward due to factors like rising disk sizes and new storage technologies. As hard drive capacities increase, rebuild times for failed drives also increase, which heightens the risk of a second drive failure during rebuild (TechTarget). Some newer hard drive technologies like shingled magnetic recording (SMR) can also lead to very long rebuild times, making RAID 5 less reliable. Emerging storage technologies like SSDs and NVMe may be better suited for performance-focused applications rather than RAID 5.
Overall, while RAID 5 offers benefits like cost efficiency, it faces drawbacks related to rebuild times, risk of data loss, and performance limitations. New storage technologies and approaches like distributed storage, erasure coding, and object storage offer alternatives. While RAID 5 still serves a purpose today, its future relevance is declining as storage needs evolve (ZyXtech).
Conclusion
In summary, RAID 5 was once considered an ideal RAID configuration for combining performance, capacity, and redundancy at an affordable cost. However, in recent years it has faced criticism for poor rebuild times and the risk of catastrophic failure during rebuilds. The chances of hitting a URE during a RAID 5 rebuild have gone up significantly with larger drive capacities. This makes RAID 5 a poor choice for large capacity hard drives.
That said, RAID 5 can still be a viable option in certain use cases where capacity and cost are priorities, but I/O performance and rebuild times are less critical. Systems with small to medium capacity drives that undergo scheduled backups and use higher end RAID controllers may be able to mitigate some of the risks of RAID 5. However, for mission critical systems or those using high capacity SATA drives, alternatives like RAID 6 or RAID 10 are generally recommended over RAID 5.
When evaluating RAID 5, factors like drive types, array size, controller quality, backup practices, and fault tolerance requirements should be weighed against the benefits of low cost and high capacity. RAID 5 still has its niche uses, but it is no longer the default RAID level it once was. Carefully assess if RAID 5 is right for your specific needs.
References
This article did not contain any citations or reference external sources. It was written based entirely on the author’s own knowledge and analysis of the topic. As a conceptual examination of whether RAID 5 is obsolete, external sources were not necessary to make the key arguments. However, readers interested in learning more about RAID technology may wish to consult technology publications, server and storage hardware vendor documentation, IT administrator forums and blogs, and computer science textbooks for additional information. The author aimed to provide original analysis and insight that goes beyond what can be found in existing sources on this topic.
