RAID (Redundant Array of Independent Disks) is a technology that combines multiple hard drives together to improve performance and/or reliability (https://www.techtarget.com/searchstorage/definition/RAID). The main purposes of RAID are to protect against disk failures and improve I/O performance. There are several standard RAID levels, each with its own benefits:
- RAID 0 stripes data across multiple disks for faster reads and writes. However, it offers no redundancy.
- RAID 1 mirrors disks for complete data redundancy but no performance gain.
- RAID 5 distributes parity information across disks so that data can be recovered if a disk fails. It provides redundancy while also improving performance.
By combining multiple physical disks into logical units, RAID aims to improve performance, capacity, and/or reliability compared to single drives. The different RAID levels balance these factors differently depending on the needs of the storage system (https://www.diskinternals.com/raid-recovery/benefits-of-raid/).
What is RAID 5?
RAID 5 is a type of RAID (Redundant Array of Independent Disks) that provides data redundancy and fault tolerance by using distributed parity. In RAID 5, data is striped across multiple disks like in RAID 0, but it also writes parity information distributed across all the disks (TechTarget, 2022). The parity allows for data recovery in case one of the disks fails.
RAID 5 requires a minimum of three disks – data is striped across all disks but one disk is reserved for parity information that is distributed across all disks. This allows for one disk failure without data loss. If a disk fails, the parity information can be used to reconstruct the data from the failed disk (EaseUS, 2022).
The main benefits of RAID 5 are cost-efficiency and optimized storage capacity. Since parity information is distributed and requires only one extra disk, the storage overhead is lower compared to mirroring in RAID 1. The main disadvantage is lower performance for write operations since parity information has to be updated each time data is written.
Distributed Parity
In RAID 5, parity is distributed across all the disks in the array instead of being dedicated to a single disk like in RAID 3 and 4. This is known as distributed parity (Raid 5, raid levels, Distributed parity – What is my Computer).
Distributed parity works by calculating parity information for each stripe of data and distributing the parity across all the disks. For example, in a 3 disk RAID 5 array, disk 1 may contain parity for stripes 1, 4, 7 etc. while disk 2 contains parity for stripes 2, 5, 8 etc. and disk 3 contains parity for stripes 3, 6, 9 etc (RAID: dedicated and distributed parity).
This distribution of parity provides redundancy and fault tolerance just like dedicated parity, but avoids the bottleneck of having a single dedicated parity disk. It allows for better performance since all disks can participate in reading and writing data in parallel.
Parity Implementation in RAID 5
In RAID 5, parity information is distributed across all the drives in the array, unlike in RAID 4 where it is confined to a single dedicated drive. This distributed implementation is a key aspect of RAID 5.
Parity is calculated using the XOR (exclusive or) function. The XOR operation is performed on the corresponding bits from all the data drives, with the result stored on the parity drive. For example, in a 3 drive RAID 5 array:
Drive 1: 1 1 0 1 0 1 0 1
Drive 2: 0 1 1 0 1 0 1 1
XOR result (parity): 1 0 1 1 1 1 1 0
This parity data is then written to the dedicated parity drive. If any single drive fails, the data on it can be reconstructed by performing the XOR operation on the parity drive and the remaining good data drives [1].
By spreading parity across all drives, write operations in RAID 5 do not need to update the parity drive every time, improving performance. The distributed implementation also avoids the parity drive bottleneck inherent in RAID 4.
Advantages of Distributed Parity
One of the main advantages of distributed parity in RAID 5 is increased redundancy and elimination of a single point of failure (RAID Level 0, 1, 5, 6, 10: Advantages, Disadvantages, and …). By distributing parity information across multiple disks, RAID 5 avoids storing all parity data on a single dedicated disk. If that single disk fails, all redundancy is lost. With distributed parity in RAID 5, the failure of one disk does not result in total loss of parity data. The remaining disks still contain portions of the parity information, providing redundancy.
This distributed storage of parity allows RAID 5 to continue operating unaffected in the event of a single disk failure. The missing data from the failed disk can be recreated using the parity information from the remaining disks. This eliminates a single point of failure and provides fault tolerance (Advantages and Disadvantages of Raid Levels). By distributing parity across multiple disks, RAID 5 removes the single point of failure that dedicated parity disks introduce.
Disadvantages of Distributed Parity
While distributed parity provides fault tolerance and redundancy, it also comes with some drawbacks. Two of the main disadvantages are increased complexity and longer rebuild times.
The parity calculations required for distributed parity lead to greater complexity in RAID 5 compared to a basic RAID 0 array. The RAID controller needs to do more computations to ensure data and parity remain in sync. This extra processing can potentially slow down write speeds.
In addition, when a disk fails in a RAID 5 array, the rebuild process takes longer compared to a RAID 1 mirror. This is because the missing data has to be recalculated using the distributed parity, rather than simply copying from an existing duplicate. The larger the disks, the longer this rebuild takes. This exposes the array to risk of a second disk failure during the rebuild.
According to one analysis, rebuilds can take over 24 hours on large arrays (Liquid Web). To mitigate this issue, some recommend hot spares to reduce rebuild times. However, the inherent complexity of parity calculations remains an ongoing disadvantage of distributed parity in RAID 5.
Alternatives to RAID 5
While RAID 5 offers distributed parity and is a popular choice for many setups, there are alternatives that may be better suited depending on your needs. Some common alternatives to consider include:
RAID 6
RAID 6 is similar to RAID 5 in that it stripes data and parity across multiple drives. The key difference is that RAID 6 utilizes two parity drives instead of one. This provides additional fault tolerance and ability to withstand multiple drive failures before data loss occurs. RAID 6 can be a good option for very large arrays or mission critical data where uptime is essential. However, the tradeoff is reduced overall capacity since more disks are devoted to parity. [1]
RAID 10
RAID 10 combines both striping and mirroring of data across drives. It provides performance benefits from striping as well as redundancy from mirroring. RAID 10 can sustain multiple drive failures so long as no more than one drive fails per mirrored set. The downside is lower overall capacity since data is duplicated through mirroring. RAID 10 is a popular choice for applications requiring high performance and redundancy. [2]
Overall, factors like performance, capacity, redundancy, and budget will determine if an alternative RAID configuration may be preferable to RAID 5 for a given use case.
When to Use RAID 5
RAID 5 offers a good balance of performance, capacity efficiency, and redundancy for many use cases. According to TechTarget, RAID 5 is commonly used in storage for databases, business applications, file and application servers, and virtual machine storage.1 Some ideal scenarios to use RAID 5 include:
- Database servers that need redundancy but also benefit from distributed reads and writes across multiple disks.
- File servers holding business documents, where performance and redundancy are priorities.
- Application servers running enterprise software, where uptime is critical.
- VMware hypervisor storage, to protect against disk failure.
In general, TechGenix recommends RAID 5 for read-intensive environments where high read performance is needed. The distributed parity helps speed up reads. RAID 5 can also work well when the total storage size needs to be maximized across a certain number of disks.2
However, if top performance and redundancy are required, alternatives like RAID 10 or RAID 6 may be better choices despite the higher cost.
Summary
In summary, RAID 5 utilizes distributed parity to provide redundancy and fault tolerance. The key points are:
- RAID 5 stripes data and parity information across all drives in the array.
- Parity allows the system to reconstruct data in case of a drive failure.
- Parity is distributed evenly across the drives, unlike RAID 3 and 4.
- This distributed parity provides performance benefits over RAID 3/4.
- The tradeoff is reduced usable capacity compared to a non-redundant array.
To recap, RAID 5 provides fault tolerance through distributed parity striping. By spreading parity across drives, it avoids bottlenecks and offers better performance compared to RAID 3/4. The distributed nature of the parity in RAID 5 is a key differentiator from other redundant RAID levels.
References
List of sources cited in article:
- R. W. Hamming, “Error Detecting and Error Correcting Codes,” The Bell System Technical Journal, vol. 29, no. 2, pp. 147-160, April 1950.
- D. Patterson, G. Gibson and R. Katz, “A Case for Redundant Arrays of Inexpensive Disks (RAID),” International Conference on Management of Data (SIGMOD), June 1988.
- P. Massiglia, “The RAID Book: A Storage System Technology Handbook,” 6th Edition, RAID Advisory Board, 1997.
- M. Farley, “Building Storage Networks,” Osborne/McGraw-Hill, 2000.
- P. Chen, E. Lee, G. Gibson, R. Katz, D. Patterson, “RAID: High-Performance, Reliable Secondary Storage,” ACM Computing Surveys, vol. 26, no. 2, pp. 145-185, June 1994.
