When setting up storage systems, redundancy is a key factor to ensure data protection in case of disk failure. RAID (Redundant Array of Independent Disks) allows you to combine multiple disk drives to provide fault tolerance and improved performance. But with different RAID levels available, which one provides the best redundancy for your needs? Here are some quick answers before we dive into the details:
Quick Answers
What is RAID? RAID combines multiple disks into a logical unit for redundancy and/or improved performance. Data is distributed across the disks according to the RAID level.
What RAID levels provide redundancy? RAID levels 1, 5, 6, 10 provide redundancy by duplicating data across disks.
Which is the most redundant RAID level? RAID 10 provides the highest redundancy by mirroring all data on a second set of disks.
What are the tradeoffs of highly redundant RAID? More disks are required, reducing usable capacity. Rebuilding arrays after failure also takes longer.
What is the minimum number of disks for RAID 10? 4 disks are required to implement RAID 10.
Introducing RAID
RAID was first conceived in 1987 to address the growing need for fault tolerance as disk drives became increasingly integral for computer systems. The key goals of RAID are to provide protection against disk failures and improve I/O performance. This is achieved by combining multiple physical drives into a single logical unit and distributing data across the drives based on the RAID level used.
Some key advantages of using RAID include:
- Preventing data loss in case of disk failure – data is duplicated across disks
- Improved read and write speeds – data can be accessed in parallel
- Increased storage capacity – multiple disks add up capacity
When setting up RAID, you get to choose the RAID level that determines how data is arranged across the disks. But before getting into the levels, there are some criteria to consider when choosing RAID for your storage needs:
RAID Selection Criteria
- Desired redundancy – how much duplication for protection against disk failure?
- Number of disks – how many disks are available for the array?
- Performance needs – read/write speed requirements
- Cost – budget available for disks
- Capacity – usable space required from the array
These factors will determine which RAID level works best for your situation. Now let’s look at the key RAID levels and their redundancy capabilities.
RAID 0
RAID 0 provides no redundancy. It simply splits data evenly across all disks in the array. While it does not provide fault tolerance, it improves performance by enabling parallel disk access.
With RAID 0, if one disk fails, all data in the array is lost. The array will need to be rebuilt from backups. RAID 0 requires a minimum of two disks.
Advantages
- Faster read/write performance – data is spread evenly across disks for parallel access
- High capacity – combines capacity of all disks into a single volume
Disadvantages
- No fault tolerance – one disk failure results in total data loss
- Less reliable – higher risk of failure with each disk added
Overall, RAID 0 improves performance but is not recommended if redundancy is required. It is more suited for temporary data that is backed up elsewhere.
RAID 1
RAID 1 provides full redundancy through disk mirroring. Data is duplicated on a second disk to provide fault tolerance. If one disk fails, data can still be accessed from the mirror.
RAID 1 requires a minimum of 2 disks for mirroring. Each write requires both disks to be updated, slowing performance. But reads can be parallelized across the disks.
Advantages
- Complete data redundancy – protection against single disk failure
- Simple configuration – only two disks needed
Disadvantages
- Slow write performance – writes must update both mirrored disks
- High cost – requires at least 2x storage capacity
- Limited capacity – only half the total capacity is usable
RAID 1 is suited for mission critical data that requires full redundancy with a minimal disk footprint. But the high cost and slow writes make it less ideal for high capacity or speed dependent applications.
RAID 5
RAID 5 provides redundancy through parity. Parity information is spread evenly across all disks, enabling recovery from the failure of any one disk.
RAID 5 requires a minimum of 3 disks. The parity data occupies disk capacity, reducing overall usable space. But it allows parallel reads across all disks.
Advantages
- Single disk fault tolerance
- Good read performance – load balancing across disks
Disadvantages
- Slower writes – parity must be updated on each write
- Loss of array capacity to parity (n-1) disks usable
- Long rebuild times after drive failure
RAID 5 provides a balanced option combining redundancy with good performance. But write speeds and rebuild times suffer compared to mirroring. RAID 6 addresses this through double distributed parity.
RAID 6
RAID 6 provides stronger redundancy than RAID 5 through double distributed parity. This allows the array to survive the loss of two disks.
At least 4 disks are needed for the additional parity. This further reduces overall usable capacity. But RAID 6 offers the highest level of redundancy below mirroring.
Advantages
- Survives up to two disk failures
- Read performance comparable to RAID 5
Disadvantages
- Slow write performance due to double parity
- Lowest usable capacity due to dual parity disks
- Long rebuilds with larger disk counts
RAID 6 works best for larger arrays where the loss of two disks is a concern. But for lower disk counts, RAID 10 provides similar redundancy with faster performance.
RAID 10
RAID 10 provides full redundancy through mirroring, similar to RAID 1. But it mirrors two RAID 0 arrays to improve parallelism. This combines RAID 1 and 0 benefits.
At least 4 disks are required for RAID 10, configured as two mirrored pairs. Read and write performance are enhanced by spreading I/O across disks.
Advantages
- Full redundancy through mirroring
- Fast read and write performance
Disadvantages
- High disk requirements – minimum 4 disks
- High cost for large capacity
RAID 10 provides the best overall performance and redundancy below pure mirroring. It is well suited for mission critical systems that require both speed and fault tolerance.
RAID Comparison
Here is a comparison of key RAID levels and their redundancy and performance characteristics:
| RAID Level | Minimum Disks | Redundancy | Read Performance | Write Performance |
|---|---|---|---|---|
| RAID 0 | 2 | None | Excellent | Excellent |
| RAID 1 | 2 | Full (mirroring) | Good | Poor |
| RAID 5 | 3 | Single disk (parity) | Good | Poor |
| RAID 6 | 4 | Double disk (dual parity) | Good | Poor |
| RAID 10 | 4 | Full (mirroring) | Excellent | Excellent |
Choosing the Right RAID Level
When choosing a RAID level, you need to balance redundancy, performance and cost considerations for your specific needs. Here are some guidelines for choosing the right RAID solution:
- For pure performance with no redundancy, RAID 0 is the best option
- For simple mirroring and minimal disk count, look at RAID 1
- RAID 5 provides good balance of redundancy and efficiency for most uses
- For high redundancy with large arrays, RAID 6 can survive up to 2 disk failures
- RAID 10 is the optimal choice for both redundancy and performance
How the array will be used should drive your RAID selection. Online transaction systems and databases demand both redundancy and speed, making RAID 10 ideal. Media servers streaming high bandwidth content can get by with just RAID 5 or 6. And backup disks may only require RAID 1 mirroring.
Conclusion
The RAID level you choose will depend on your specific priorities – redundancy, performance, capacity, and cost. If your main goal is to prevent disk failure and downtime, opt for high mirroring with RAID 1 or 10. When budget constraints are important, RAID 5 provides a good balance with single parity protection. And if you have high capacity disks with lower failure rates, RAID 6 can provide an efficient double parity solution.
Think carefully about your use case and criteria such as number of disks, I/O patterns, and acceptable failure rates. This will lead you to pick the optimal RAID solution for building a reliable, high performance storage system.