How many RAID 5 disks can fail?

RAID 5 is a disk or solid state drive (SSD) subsystem that increases safety by computing parity data and increasing performance by distributing read and write load across multiple disks ( It is the most popular RAID configuration because it provides a good balance between data protection, performance, and storage efficiency. The key characteristics of RAID 5 are:

  • Data and parity information are striped across 3 or more disks
  • If any 1 disk fails, data can be rebuilt using the parity disk
  • Performance is improved by spreading reads and writes across multiple disks
  • Storage efficiency is good since only 1 disk worth of space is used for parity

RAID 5 provides protection against a single disk failure, which is the most common scenario. However, rebuilding a failed drive can put substantial stress on the system. RAID 5 performs well for read operations since load is balanced, but write performance suffers due to parity calculation. Overall, RAID 5 offers a versatile option that works well for many use cases if redundancy and moderate performance are priorities.

RAID 5 Architecture

A typical RAID 5 setup consists of at least three disks, with one disk’s worth of space used for parity information (also called the parity disk). The remaining disks are used for data storage and are striped, meaning the data is broken down into blocks and distributed among the data disks in chunks. The parity information is also distributed among the disks, with each disk holding a portion of the parity data calculated from the data blocks on the other disks. This is known as distributed parity.

For example, in a 3-disk RAID 5 array, Disks 1 and 2 might hold data blocks A1, A2, B1 and B2 while Disk 3 would hold parity blocks P1 and P2 calculated from A1+A2 and B1+B2 respectively. On the next stripe, Disk 3 might hold data blocks C1 and C2 while Disks 1 and 2 would hold parity information calculated from the data on Disk 3. This alternating parity stripe distributes the parity information evenly across all disks. The distributed parity along with the striped data provides redundancy and protection in case of a single disk failure (see

Disk Failure in RAID 5

RAID 5 is designed to withstand the failure of one disk drive in the array without losing data. When a single disk fails, the array will continue operating normally using the parity information to reconstruct the missing data from the failed drive. The array will switch into a degraded mode until the failed drive is replaced and the data is rebuilt onto the new drive.

While RAID 5 can survive a single disk failure, the array is still vulnerable during the rebuild process. If a second disk fails before the rebuild is complete, the entire array will fail and all data will be lost. For this reason, it is crucial to replace the failed drive and complete the rebuild as quickly as possible.

The rebuild process reads all the data blocks from the surviving disks and uses XOR calculations against the parity block to reconstruct the missing data from the failed drive. This process puts additional stress on the surviving disks as they are being read constantly during the rebuild. The larger the disks and the more data stored, the longer the rebuild takes and the greater the risk of a second disk failure.

According to ServerFault, if a second disk fails before the rebuild is complete, specialized data recovery software is required to recover the RAID 5 array. Software like R-Studio and Zero Assumption Recovery use advanced algorithms to reconstruct the array from the two failed disks.

Single Disk Failure

RAID 5 can survive a single disk failure without data loss. This capability comes from the parity information that is distributed across all the disks in the RAID 5 array. If a single disk fails, the parity information on the remaining disks can be used to reconstruct the data that was on the failed disk.

When a single disk does fail in a RAID 5 array, the system will switch into a degraded mode and a hot spare disk will automatically start rebuilding the data from the failed disk. If there is no hot spare, the system will wait for the failed disk to be replaced before initiating a rebuild. The rebuild process reads all the data from the surviving disks and uses XOR calculations with the parity information to reconstruct the data that was on the failed disk. The rebuild is done disk block by disk block until the entire failed disk has been recreated onto the replacement disk. The time for rebuild depends on the size of the disks and performance of the controller, but could take several hours to complete on large arrays.1

Double Disk Failure

The main vulnerability of RAID 5 arrays is double disk failure, which causes unrecoverable data loss. With single disk parity and striping, RAID 5 cannot reconstruct data if two disks fail [1]. The array ceases to be fault tolerant and all data is lost. This is a catastrophic failure.

RAID 5 is only designed to handle a single disk failure because it uses single parity. Single parity means there is enough redundant information to reconstruct data if one disk fails. But with two failed disks, the data on both disks is permanently lost. There is not enough redundant data to recover from two disk failures [2].

RAID 6 vs RAID 5

RAID 6 and RAID 5 are similar RAID types that store data with parity information for redundancy. However, RAID 6 offers greater fault tolerance by using dual distributed parity compared to the single parity of RAID 5. This allows RAID 6 to survive up to 2 concurrent disk failures without data loss, while RAID 5 can only handle a single disk failure.

The dual parity in RAID 6 provides an extra layer of protection, but comes at the cost of reduced storage efficiency. RAID 6 requires a minimum of 4 disks, with 2 disks worth of capacity used for parity information. This leads to about half the usable capacity compared to an equivalent RAID 5 array. RAID 6 also has reduced write performance due to the extra parity calculations required on writes.

For mission critical data that requires high availability, the extra fault tolerance of RAID 6 is preferable despite the capacity and performance tradeoffs. However, for less critical data, RAID 5 offers a better balance of storage efficiency, performance, and redundancy for many use cases.

Best Practices

When using RAID 5, it is important to follow best practices to maximize reliability and performance. According to a Dell technical whitepaper (source), the key best practices are:

  • Use RAID 6 instead of RAID 5 for better redundancy against double disk failures.
  • Configure hot spares to allow quick rebuilding if a disk fails.
  • Monitor disk health closely to identify potential failures before they happen.

By following these best practices, organizations can build reliable and high-performance RAID 5 arrays that minimize disruption from failed disks.

Software vs Hardware RAID 5

Software and hardware implementations of RAID 5 each have their advantages and disadvantages in terms of performance, flexibility, cost, and ease of use:

Software RAID 5 runs as a driver in the operating system, without needing specialized RAID hardware controllers. This makes it less expensive to implement, and allows for more flexibility in configuring arrays across different types of disks. However, software RAID incurs additional CPU overhead that can impact performance, especially during peak utilization or RAID operations like rebuilds. Software RAID 5 may also have limitations in features compared to hardware controllers.

Hardware RAID 5 uses dedicated disk controllers with on-board processors to handle the RAID calculations and parity. This offloads the CPU and can provide substantial performance improvements over software RAID. Hardware RAID also offers advanced caching techniques to optimize reads and writes. However, hardware RAID controllers add significant costs over software RAID. There are also limitations in flexibility, scalability, and portability when tying RAID to a physical controller.

In general, hardware RAID 5 provides faster peak performance while software RAID offers more flexibility and lower costs. For mission-critical storage, hardware RAID 5 is usually preferable, especially in busy environments. But software RAID 5 can be sufficient for many general-purpose workloads. The choice often depends on budget, performance needs, availability of hardware, and ease of management.


Newer Alternatives

Organizations seeking more fault tolerance and reliability than RAID 5, without the performance penalty of RAID 6, can consider newer systems such as erasure coding and distributed non-RAID storage. Erasure coding can provide protection beyond the RAID 5 or RAID 6 threshold, by distributing and encoding data across more disks. This allows for simultaneous failures of multiple disks without data loss (cite:[2]). Major cloud providers like AWS S3 store customer data using erasure coding for availability and durability.

Other options beyond traditional RAID include distributed file systems and object storage, which spread data across nodes without striping or parity calculations. These systems rely on massive redundancy across a storage cluster to protect against failures, without the dedicated parity disks of RAID 5/6. Products like Ceph provide object storage with configurable data protection schemes to survive multiple disk or node losses (cite:[1]). As data needs grow, alternatives to RAID 5 provide higher capacity, performance, and fault tolerance.


In summary, RAID 5 arrays provide redundancy by striping data and parity information across multiple disks. The key points around disk failures in RAID 5 include:

– RAID 5 can withstand a single disk failure without data loss. If one disk fails, the parity information on the other disks can be used to reconstruct the missing data.

– However, RAID 5 arrays are vulnerable to a second disk failure during the rebuild process after the first disk fails. If a second disk fails before the array finishes rebuilding, data loss will occur.

– For better redundancy, many recommend moving to RAID 6 instead of RAID 5. RAID 6 can survive up to two disk failures by using a second set of parity information.

In summary, RAID 5 allows a single disk failure, but for higher availability, consider migrating to RAID 6 which can tolerate up to two disk failures.