What Is Parity in Raid 5 Explained
RAID 5 is a redundant array of independent disks configuration that uses disk striping with parity. Because data and parity are striped evenly across all of the disks, no single disk is a bottleneck. Striping also allows users to reconstruct data in case of a disk failure.
RAID 5 evenly balances reads and writes, and is currently one of the most commonly used RAID methods. It has more usable storage than RAID 1 and RAID 10 configurations, and provides performance equivalent to RAID 0.
RAID 5 groups have a minimum of three hard disk drives (HDDs) and no maximum. Because the parity data is spread across all drives, RAID 5 is considered one of the most secure RAID configurations.
How RAID 5 works
The benefits of RAID 5 primarily come from its combined use of disk striping and parity. Striping is the process of storing consecutive segments of data across different storage devices, and allows for better throughput and performance. Disk striping alone does not make an array fault tolerant, however. Disk striping combined with parity provides RAID 5 with redundancy and reliability.
RAID 5 used parity instead of mirroring for data redundancy. When data is written to a RAID 5 drive, the system calculates parity and writes that parity into the drive. While mirroring maintains multiple copies of data in each volume to use in case of failure, RAID 5 can rebuild a failed drive using the parity data, which is not kept on a fixed single drive.
By keeping data on each drive, any two drives can combine to equal the data stored on the third drive, keeping data secure in case of a single drive failure. Drives can be hot swapped in RAID 5, which means a failed HDD can be removed and replaced without downtime.
RAID 5 layout
At least three drives are required for RAID 5. Depending on where the parity blocks are located and in which order the data blocks are written, there can be four different types of RAID 5. Left and right arraysare determined by how the parity blocks are distributed onto the member disks. Synchronicity and asynchronicitydefine the order of the data blocks.
Advantages
RAID 5 is one of the most common RAID configurations and is ideal for application and file servers with a limited number of drives. Considered a good all-around RAID system, RAID 5 combines the better elements of efficiency and performance among the different RAID configurations.
Fast, reliable read speed is a major benefit. This RAID configuration also offers inexpensive data redundancy and fault tolerance. Writes tend to be slower, because of the parity data calculation, but data can be accessed and read even while a failed drive is being rebuilt. When drives fail, the RAID 5 system can read the information contained on the other drives and recreate that data, tolerating a single drive failure.
Disadvantages
Longer rebuild times are one of the major drawbacks of RAID 5, and this delay could result in data loss. Because of its complexity, RAID 5 rebuilds can take a day or longer, depending on controller speed and workload. If another disk fails during the rebuild, then data is lost forever.
Additionally, while the backup provided by RAID configurations can contribute to business continuity, they are not equivalent to a disaster recovery setup in which hardware is locate in both on-premise and off-premise locations.
Software vs. hardware RAID
RAID can be in the form of hardware or software, depending on where the processing occurs. Software RAID is a form of RAID performed on the internal server. Because it processes on the internal server, software RAID is slower than hardware RAID. However, because hardware RAID requires purchasing additional hardware, software RAID costs less.
Popularity vs. other types of RAID configurations
All RAID configurations offer benefits and drawbacks. Standard RAID levels such as 2, 3, 4 and 7 are not as commonly used as others, such as 5, 1, 6 and 10. While RAID 3 could be considered inferior to RAID 5 because it uses a separate disk for parity data, other configurations can hold their own when compared to RAID 5.
RAID 1 writes to two mirrored disk drives, and can handle twice the number of reads than a single HDD. This has kept RAID 1 as one of the most favored configurations and, in terms of speed, it can outperform RAID 5. However, the amount of disk space required by RAID 1 can make RAID 5 a more appealing option. RAID 1 also has slower write speeds than 5. RAID 1 can still be a good choice in settings where data loss is unacceptable, such as data archiving.
Similar to RAID 5, RAID 6 has speedy reads and writes parity data to multiple drives. However, because it writes to two drives, RAID 6 uses a minimum of four drives rather than the three required by RAID 5. Unlike RAID 5, RAID 6 can withstand two drive failures and provide access to all data even while both drives are being rebuilt. Because of this, RAID 6 is considered more secure than RAID 5.
With RAID 6, writes are even slower than RAID 5 because of the additional parity data calculation. Similar to RAID 5, while data is still accessible while a drive is being rebuilt, rebuilds can take a considerable amount of time. RAID 6 is considered an all-around solid system, and may be preferable to RAID 5 in environments where a high number of large drives are used for storage.
RAID 10, or RAID 1+0, is a nonstandard RAID configuration that combines elements of RAID 1 and RAID 0. Unlike RAID 5 and RAID 6, RAID 10 has a fast rebuild time, thanks to the ability to copy mirrored data to a new drive. This process can take as little as 30 minutes, depending on the drive size. The drawback to RAID 10 is that half of all storage capacity goes to mirroring, which can speed up rebuilds but can become expensive quickly.
Trends and future directions
Despite the numerous configurations available, RAID is an aging technology that is facing off with new competitors in the storage space. However, many vendors are beginning to use RAID to supplement technologies like solid-state drives (SSDs) to give them redundancy. Until a more reliable form of data redundancy becomes available, RAID will likely continue to have a place in the storage market.
While RAID 5 remains popular, other RAID schemes have their selling points. The ability of RAID 6 to withstand two drives failing makes it an appealing option, and disk vendors are recommending RAID 6 and 10 for larger workloads. Standard SATA drives are not a good fit for RAID 5, because administers can be prevented from rebuilding a drive after a failure.
Storage capacity growth is another factor to watch when considering the future of RAID 5. As HDD sizes increase, RAID 5 rebuild times will only rise, and put the system at risk for another drive failing in that time. An increase in storage density that isn't met by better performance will result in a lengthy rebuild. And with so many variations of RAID available to fix the mistakes of earlier configurations, better options are likely to appear down the road.
This was last updated in July 2018
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What Is Parity in Raid 5 Explained
Source: https://searchstorage.techtarget.com/definition/RAID-5-redundant-array-of-independent-disks