RAID 5 is a type of redundant array of independent disks (RAID) that uses distributed parity to provide data protection and fault tolerance while also providing improved performance compared to a single disk. RAID 5 requires a minimum of three disks, but can scale to larger arrays with additional disks.
Quick Answers
– At least 3 physical hard drives are needed for RAID 5
– RAID 5 uses distributed parity, storing parity data across all the drives
– If one drive fails, data can be rebuilt using the parity data
– RAID 5 provides good performance and efficient use of disk space
Some key advantages of RAID 5:
– Data protection against single disk failure
– Good read performance since data is striped across multiple disks
– Efficient use of storage capacity compared to mirroring
– Standard and widely supported RAID level
Some disadvantages:
– Slower write performance due to parity calculation
– The array is vulnerable during rebuild after a disk failure
– Not ideal for random write workloads
RAID 5 Fundamentals
RAID 5 is based on block-level striping with distributed parity. This means that data is broken down into blocks which are written across multiple disks in the array. Additionally, parity information is calculated and written across the disks.
The distributed parity provides redundancy and protection against disk failure. If one of the disks in the RAID 5 array fails, the missing data can be recreated using the parity data from the surviving disks.
For example, in a 3-disk RAID 5 array, here’s how data and parity blocks may be distributed:
| Disk 1 | Data A |
| Disk 2 | Data B |
| Disk 3 | Parity A+B |
The parity information is rotated across different disks for each stripe. This avoids the parity disk becoming a bottleneck.
Minimum Disk Requirements
To implement RAID 5, you need **at least 3 physical disks**. This provides the minimum disks needed for distributed parity and protection against a single disk failure.
With 3 disks, the total storage capacity is the sum of 2 disks, as 1 disk’s worth of capacity is used for parity.
The disks in a RAID 5 array do not all need to be the same size, but storage space equivalent to the smallest disk will be lost. For best performance and space efficiency, it is ideal to use disks of the same size.
While RAID 5 can technically be implemented with 2 disks by using disk mirroring for parity, this setup does not provide the performance and capacity benefits of true RAID 5 with distributed parity.
More Disks for Larger Arrays
For larger arrays, RAID 5 can scale to use more disks. Using additional disks increases the total storage capacity of the array.
With 4 disks, there is usable capacity equal to 3 disks. With 5 disks, there is capacity equal to 4 disks, and so on.
Each disk added improves the overall performance by increasing the number of spindles available for parallel I/O. More disks also provide the ability to withstand additional disk failures.
Large RAID 5 implementations often use a dedicated hot spare disk that can automatically replace a failed drive to start rebuilding faster.
Distributed Parity in RAID 5
A key characteristic of RAID 5 is the use of distributed parity across multiple disks. Parity allows the array to withstand a single disk failure by providing redundancy.
How Parity Works
Parity is calculated by performing an exclusive OR (XOR) operation on the bits of data being written. The result is the parity.
For example:
| Data Block A | 10101010 |
| Data Block B | 11010101 |
| Parity A+B | 01100111 |
If a disk was to fail, the parity block could be used to reconstruct the missing data block by reversing the XOR operation.
Distributing Parity Across Disks
Rather than storing all parity on a single disk, RAID 5 distributes parity across all the disks.
This avoids the parity disk becoming a bottleneck during writes. By distributing parity, all disks can participate in I/O operations in parallel.
Each stripe of data uses a different disk for its parity block. This results in the parity being evenly distributed or “striped” across the array.
RAID 5 Performance Characteristics
RAID 5 provides overall improved performance compared to a single disk, as well as efficient use of disk capacity. However, its performance for certain workloads is more nuanced.
Reads
For read operations, RAID 5 provides very good performance. Since data is striped across all disks, the workload can be distributed across multiple disks allowing parallel disk I/O.
The more disks present, the better the overall read performance scales.
Writes
Write performance is more complex with RAID 5. Because parity must be calculated and written with each stripe, the write process takes longer.
However, since this is distributed across disks, multiple writes can still occur in parallel. Performance is much better compared to writing to a single disk.
That said, RAID 5 performs better with larger stripe sizes. When writing small random blocks, performance suffers due to frequent parity recalculation.
Rebuilds
When recovering from a failed disk, data must be rebuilt based on the parity blocks. This rebuild process temporarily reduces performance until complete.
Larger arrays take longer to rebuild, which exposes the array to risk of a second disk failure during this time.
Caching and Battery Backup
To improve write performance, RAID controllers often use write-back caching as well as battery backup units (BBUs).
Caching allows faster writes by temporarily storing data in memory before writing to disk. BBUs protect cached data during power loss.
Choosing RAID 5 Over Other RAID Levels
There are several standard RAID levels, each with their own performance and fault tolerance tradeoffs. RAID 5 provides a balanced option appropriate for many use cases.
RAID 0
Compared to RAID 0 which stripes data across disks for pure performance, RAID 5 provides fault tolerance by using parity.
RAID 0 has no redundancy making it risky, while RAID 5 protects against disk failure.
RAID 1
Unlike RAID 1 which mirrors data across disks, RAID 5’s distributed parity allows better use of capacity.
Where RAID 1 might require 4 disks to provide capacity of 2 disks, RAID 5 provides 3 disks of capacity from 4 disks.
RAID 6
RAID 6 adds an additional parity disk to provide protection against failure of two disks. This comes at a write performance penalty.
For most use cases, the additional cost and reduced performance of RAID 6 is unnecessary beyond RAID 5’s protection against a single disk failure.
RAID 10
RAID 10 combines mirroring and striping for both performance and redundancy. However, the capacity overhead is very high.
RAID 5 provides Fault tolerance while allowing far more efficient use of disk capacity compared to RAID 10.
Ideal Uses Cases for RAID 5
Given its performance and redundancy capabilities, RAID 5 excels in a number of storage use cases:
– File servers and network attached storage (NAS) devices where reads are frequent compared to writes.
– Database servers that involve transactional workloads with a mix of reads and writes.
– General purpose servers that require good disk performance and Medium to high capacity with fault tolerance.
– Video surveillance storage where video is appended continuously to a large storage pool.
– Media servers that store large files like videos, photos, audio files, etc.
When Not to Use RAID 5
RAID 5 is not well suited to certain workloads:
– High volume random write environments, like busy transactional databases. The parity overhead on writes will result in poor performance.
– Small capacity arrays. Due to the fixed parity overhead, RAID 5 is inefficient for small number of disks.
– Mission critical systems where uptime is paramount. The vulnerability during rebuilds may be unacceptable for critical data. RAID 6 or 10 may be preferable.
– Applications needing sustained write performance. Workloads with long periods of non-stop writes will be bottlenecked by the parity calculation overhead.
Implementing RAID 5
There are several options available for implementing RAID 5 storage based on required scale and budget.
Hardware RAID Controller
A dedicated hardware RAID controller provides full-featured RAID management. The controller handles all parity calculations and abstraction of the underlying disks.
Hardware RAID provides high performance and advanced caching capabilities. Most enterprise servers rely on hardware RAID.
However, hardware RAID controllers add cost to a system. they also lack flexibility and portability.
Software RAID
Many operating systems like Linux, Windows, etc. have built-in software RAID capabilities. This allows creating RAID volumes using the OS tools.
Software RAID provides a low cost option, while still providing access to RAID 5 functionality. Performance depends on the host system’s resources.
Software RAID volumes are fully portable between systems using the same software RAID application. But they require the host OS to be running to function.
RAID Cards and HBAs
Simple RAID cards provide RAID 5 support without a full hardware RAID processor. They perform parity calculations using the host system’s CPU.
Alternatively, simple host bus adapter (HBA) cards can allow access to physical disks for software RAID implementations.
These options provide cost-effective middle ground between hardware and software RAID.
Conclusion
To summarize, implementing RAID 5 requires:
– At least 3 physical disks for distributed parity
– A RAID controller (hardware or software) that supports RAID 5
– Enough storage capacity for meaningful use of RAID 5 based parity and performance benefits
The distributed parity in RAID 5 provides protection against disk failure while delivering strong read performance. This makes it a versatile, balanced RAID level suitable for a variety of workloads and storage server implementations.
Care should be taken to select the right RAID option based on capacity, performance and redundancy requirements. But for general purpose use, RAID 5 hits a sweet spot combining efficient use of disk space with fault tolerance.