What is RAID 5?
RAID 5 is a storage technology that combines distributed parity and data striping over multiple hard disk drives (HDD) or solid-state drives (SSDs) in order to provide fault tolerance and improve performance (Definition of RAID 5). Unlike RAID 1 which makes an exact copy of data on another drive, RAID 5 distributes parity information across all the member disks.
RAID 5 requires a minimum of 3 drives to implement. Data is split up into stripes and written across the drives in the array sequentially. In addition to the data stripes, parity information is also calculated and written across the drives. The parity stripes are staggered so that the parity is evenly distributed across all drives (RAID-5 volume). If any single drive fails, the parity information can be used to reconstruct the data from that failed drive.
Compared to RAID 1, RAID 5 is more storage efficient as it doesn’t require full duplication of data. And compared to RAID 0, it provides fault tolerance through the parity mechanism. However, RAID 5 performs slower for write operations than RAID 0 or 1 due to the parity calculation (RAID 5 parity bits – recovering data). Overall, RAID 5 provides a good balance of performance, fault tolerance, and storage efficiency for many applications.
How RAID 5 Stripes Data Across Drives
RAID 5 stripes data across multiple drives at the block level, similar to RAID 0. This means consecutive blocks of data are written in a sequential manner across the drives in the array. For example, block 1 is written to drive 1, block 2 to drive 2, and so on until all drives have an equal number of data blocks.
The key difference compared to RAID 0 is that RAID 5 also generates and writes parity information. Parity is calculated using an XOR operation across the data blocks in the stripe. The result of the parity calculation is written to a dedicated parity drive in the array. This parity block can be used to reconstruct data in case of a single drive failure.
For each stripe, the parity drive rotates so that parity is evenly distributed. This is known as distributed parity. With distributed parity, RAID 5 avoids concentrating all parity information on a single drive. Overall, this block-level striping and distributed parity provides redundancy and protection but with less overhead compared to mirroring in RAID 1.
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RAID 5 Storage Overhead
RAID 5 utilizes distributed parity, meaning the parity information is spread across all the drives in the array. As a result, RAID 5 requires a minimum of 3 drives to operate. The tradeoff for this distributed parity is that RAID 5 has significant storage overhead compared to a basic disk array.
Specifically, RAID 5 uses the equivalent of 1 drive’s worth of capacity for parity storage. For example, in a 3 drive RAID 5 array with 1TB drives, the total raw capacity is 3TB but the usable capacity is only 2TB due to the 1TB overhead for parity. In a 4 drive RAID 5 array with 1TB drives, the raw capacity is 4TB but the usable capacity is 3TB. This parity overhead remains constant regardless of the size of the array.
The parity overhead can be calculated as:
Total RAID 5 Storage Overhead = (Total Drives – 1) / Total Drives
Or for a 4 drive RAID 5 array:
(4 Drives – 1) / 4 Drives = 3/4 = 25% Overhead
So in summary, RAID 5 provides fault tolerance through distributed parity but at the cost of 1 drive’s worth of capacity across the size of the array. This overhead percentage remains constant as you scale the number of drives.
RAID 5 Rebuild Process
When a drive fails in a RAID 5 array, the rebuild process works to restore redundancy and protect against data loss. The RAID controller uses the parity data spread across the remaining drives to reconstruct the data that was on the failed drive and write it to a replacement drive.
This rebuild process has an impact on performance. As the RAID controller reads all the data blocks and parity from the surviving drives to reconstruct the lost data, it creates significant additional load on the array. All the drives must be read to rebuild each stripe of data that was on the failed drive. This degrades the performance of applications accessing the array during the rebuild.
According to HP, the RAID 5 rebuild process restores the failed drive one or more stripes at a time, so the read and write operations occur at a slower rate compared to normal operation[1]. Rebuild times vary based on the size of the array and controller performance, but often take several hours or days to complete.
To minimize impact, the rebuild process can be throttled to use less system resources. However, this extends the total rebuild time. Monitoring the rebuild status and limiting heavy disk activity during critical rebuild periods can help optimize RAID 5 performance.
Optimizing RAID 5 Performance
There are a few key factors that impact RAID 5 performance that can be optimized:
Chunk or stripe size is an important configuration setting when creating a RAID 5 array. The chunk size controls how data is written across the drives in stripes. A smaller chunk size leads to more even distribution of reads and writes across drives, improving performance. But it also leads to more parity calculations that can slow writes. Larger chunk sizes reduce parity overhead but can lead to uneven load balancing. A good rule of thumb is to use larger chunk sizes for arrays focused on reads and smaller sizes for write-heavy workloads. 64KB or 128KB chunks offer a good balance for general use [1].
The RAID controller hardware also impacts performance significantly. Controllers with larger caches can buffer more write operations before committing them to disk. Battery backups and flash caches prevent cache loss during power failure. More powerful RAID controller processors speed up parity calculations. Upgrading to a controller designed for performance can provide major gains.
Software vs. hardware RAID also impacts speeds. Hardware RAID offloads the bulk of RAID calculations to dedicated processors on the controller. Software RAID relies on the system CPU, adding load. Hardware RAID generally provides faster operation. But software RAID can offer more flexibility for advanced configurations.
In addition, drive types make a difference. SSDs provide faster throughput, especially random reads/writes critical for parity calcs. But they come at a higher cost per GB. Using SSDs for caching or a separate log drive can boost speed while minimizing cost.
When to Use RAID 5
RAID 5 is best suited for scenarios where storage capacity and throughput are more important than fault tolerance. According to EaseUS, RAID 5 is a good option for archival storage, where large amounts of data need to be stored cost effectively and performance needs are moderate.
RAID 5 can maximize storage capacity using the minimum number of disks, while still providing performance benefits from striping data across drives. For example, four 1TB drives in a RAID 5 can provide 3TB of usable storage, compared to just 2TB in a RAID 1 mirror. The tradeoff is lower fault tolerance than RAID 6 or 10.
RAID 5 performs well for workloads that are focused on throughput of large files, like media editing or scientific computing. The distributed parity allows all drives to participate in reads and writes. This provides better speed than RAID 1 or 10, which have to write duplicate data to mirrors or stripes. RAID 5 throughput can actually surpass RAID 0 since it avoids simultaneous writes to multiple disks.
According to a Reddit user, RAID 5 can be suitable for arrays of up to around 8 disks. Beyond that, the risk of rebuild failures increases, so RAID 6 is preferred.Overall, RAID 5 offers a balance of storage efficiency, performance, and redundancy for less critical data.
Alternatives to RAID 5
While RAID 5 has been a popular choice for many years, some alternatives have emerged that offer greater redundancy and performance. Two of the most common alternatives are RAID 6 and RAID 10.
RAID 6 provides double distributed parity like RAID 5, but uses two separate parity blocks rather than one. This allows RAID 6 to sustain up to two disk failures without data loss. The tradeoff is greater storage overhead, as two disks are required for parity instead of one. RAID 6 is a good option for large arrays where the risk of multiple disk failures is higher (TechTarget).
RAID 10 provides fault tolerance by striping data across mirrored pairs of disks. This provides faster read performance than RAID 5, as data can be read in parallel from both disks in the mirror. However, RAID 10 also requires more disks to provide the same usable capacity. RAID 10 is a good option when performance and redundancy are critical (TechTarget).
In general, both RAID 6 and RAID 10 provide stronger redundancy than RAID 5, though at the cost of usable capacity and potential performance tradeoffs. The choice between the two mainly depends on whether improved performance (RAID 10) or capacity efficiency (RAID 6) is more important for the use case (Spiceworks).
Migrating from RAID 5
Migrating from a RAID 5 configuration to a different RAID level or setup requires careful planning and execution. Here are some key considerations when migrating away from RAID 5:
To start, you’ll need enough free space available to temporarily store all the data from the RAID 5 array during the migration process. This usually means adding extra drives to the system temporarily. It’s also a good idea to have a full backup available before beginning.
The process typically involves creating a new RAID array with the desired configuration, then moving the data from the RAID 5 drives onto the new array. This can be done drive-by-drive to minimize downtime. Some RAID controllers also support converting from one RAID level to another without needing to move the data off the drives.
When migrating to RAID 6, the process is simplified since RAID 6 is an extension of RAID 5 that adds a second parity drive. Some RAID controllers can transform a RAID 5 array into RAID 6 by simply adding the extra parity drive.
Migrating from RAID 5 to RAID 10 or RAID 1+0 requires more planning, as those levels use mirroring instead of parity. The data will need to be fully copied to the new mirrored drives.[1]
On NAS devices, it’s usually possible to migrate RAID levels through the interface, but this can be a slow process for large arrays. Direct disk migration may be faster.
Testing the new array before migrating production data is highly recommended. And care should be taken to closely monitor the RAID status during and after migration.
RAID 5 in the Cloud
RAID 5 is commonly implemented in public cloud environments like AWS, Azure, and GCP to provide redundancy and improve performance for cloud-based storage. The flexibility of cloud computing allows RAID 5 to be implemented in innovative ways.
AWS offers RAID 5 as an option for EBS volumes, which can emulate traditional hardware RAID configurations. Azure supports RAID 5 in managed disks through their Storage Spaces technology. GCP allows RAID 5 to be configured using software RAID on Compute Engine persistent disks.
A key benefit of RAID 5 in the cloud is that it offers high read I/O transaction rates compared to other RAID types. Medium write I/O rates are also common.
Overall, RAID 5 gives cloud users an efficient way to optimize storage performance, redundancy, and costs across cloud servers.
The Future of RAID 5
New technologies and trends are emerging that may impact the future viability of RAID 5 configurations. As drive capacities continue to increase, the rebuild times for failed drives in RAID 5 arrays will also rise proportionally (https://www.techtarget.com/searchstorage/definition/RAID-5-redundant-array-of-independent-disks). This increases the risk of unrecoverable read errors during rebuilds and potential data loss. Some experts predict that RAID 6 may become more common than RAID 5 for these reasons.
Solid state drives (SSDs) are also playing a role in the future of RAID 5. The substantially faster write speeds of SSDs help mitigate some of the write penalty issues associated with RAID 5. However, SSDs have limited write cycles compared to hard disk drives (HDDs). The additional parity writes required in RAID 5 can more quickly consume these write cycles (https://www.zyxtech.org/2019/05/31/raid5/). For some applications, other RAID levels may be better suited for longevity and performance with SSDs.
Trends toward larger drive capacities, higher rebuild times, and growing SSD adoption are important considerations when evaluating whether RAID 5 will continue meeting needs in the future or if alternatives should be explored.