RAID (Redundant Array of Independent Disks) is a data storage technology that combines multiple disk drive components into a logical unit. RAID provides increased storage functions and reliability through redundancy.
RAID allows data to be spread across multiple disks, while at the same time it uses techniques like disk striping (RAID 0), disk mirroring (RAID 1), and parity (RAID 5) to achieve redundancy, increasing fault tolerance and improving performance.
There are two main implementations of RAID: software RAID and hardware RAID. Software RAID is implemented at the operating system level, while hardware RAID is implemented at the controller level using proprietary controllers. Both types provide benefits like improved performance and fault tolerance, but they also have differences in implementation and use cases.
What is software RAID?
Software RAID, also known as OS RAID, is implemented by an operating system’s RAID software modules. It allows the operating system to control the RAID methods like disk striping, mirroring, and parity calculations in software.
The RAID logic is handled by the OS drivers and processors. The RAID algorithms and distribution of data across disks are managed by the OS software RAID manager. Popular software RAID implementations include Linux’s mdadm and Windows Dynamic Disks.
Advantages of software RAID
- Cost – Software RAID does not require additional hardware since it uses the OS’s resources, so it is generally cheaper to implement.
- Flexibility – Software RAID allows disks from different vendors, interfaces and capacities to be used in a single RAID array.
- Portability – Software RAID arrays can be migrated between different operating systems and hardware.
- Scalability – Disks can be easily added to or removed from a software RAID array without additional hardware.
Disadvantages of software RAID
- Performance – Software RAID has CPU overhead since the processing is done by the OS, so performance may lag versus hardware RAID.
- No caching – Software RAID lacks the caching abilities of hardware RAID controllers.
- No bootability – Software RAID arrays often cannot be used as boot devices.
- OS dependence – Software RAID is tied to a specific OS platform and its software implementation.
Software RAID use cases
Software RAID is best suited for:
- Small businesses and home users who want redundancy and improved performance but don’t require high-end RAID capabilities.
- Environments where disks from different vendors or interfaces are utilized.
- Users who want to repurpose old disks as RAID arrays.
- Virtualized environments where a software RAID can be created at the VM guest level.
Software RAID is less ideal for mission critical systems that require high performance, maximum uptime and advanced RAID capabilities.
What is hardware RAID?
Hardware RAID performs the RAID calculations and logic on a dedicated hardware controller, usually on an expansion card or integrated into the system motherboard. The controller takes over the RAID tasks from the operating system.
Hardware RAID uses a RAID controller that has its own onboard processor, cache and firmware for offloading RAID tasks. The controller also provides connectivity for the disk drives, handling the formatting and drive interface protocols.
Advantages of hardware RAID
- Performance – Hardware RAID provides faster processing and more efficient data transfers due to dedicated controllers and onboard cache.
- Advanced features – Hardware RAID supports advanced features like instant hot swaps, virtualization and snapshots.
- Reliability – Hardware RAID provides redundancy and failover capabilities for enterprise-level availability.
- Boot support – Most hardware RAID arrays can be used as bootable devices.
Disadvantages of hardware RAID
- Cost – Hardware RAID controllers can be significantly more expensive than software RAID solutions.
- Less flexibility – Most hardware RAID controllers require disks of the same type and interface.
- Vendor dependence – If a controller fails, a replacement of the exact same model may be needed.
- Limited scalability – Expanding a hardware RAID array often requires identical controllers.
Hardware RAID use cases
Hardware RAID is ideal for:
- Mission critical systems that require 24/7 uptime and fault tolerance.
- Performance-sensitive applications like databases and web servers.
- Enterprise environments that need advanced management capabilities.
- Data centers and server rooms that utilize SAN storage and fiber-channel drives.
Hardware RAID is overkill for home users or small businesses that don’t require enterprise-level performance and reliability.
RAID levels
RAID can be configured in different standardized RAID levels, each optimizing for different goals – enhanced performance, redundancy or large capacities. The most common RAID levels are:
RAID 0
RAID 0 (also called disk striping) splits data evenly across two or more disks with no redundancy. RAID 0 provides improved performance by distributing the read/write workload across parallel disks but offers no fault tolerance. If any disk fails, the entire RAID 0 array will be lost.
RAID 1
RAID 1 (disk mirroring) copies and maintains identical copies of data on two or more disks. If one disk fails, the other disk can still service requests. RAID 1 provides redundancy for failure protection but halves the available capacity.
RAID 5
RAID 5 stripes data and parity information across three or more disks. If any single disk fails, the missing data can be recreated from the remaining data and parity. RAID 5 provides fault tolerance with efficient use of storage capacity. However, write performance may be slower than other RAID levels.
RAID 6
RAID 6 is similar to RAID 5 but utilizes double distributed parity instead of a single parity disk. This allows RAID 6 to sustain up to two disk failures with no data loss. However, write performance is slower and more disk capacity is used for the additional parity information.
RAID 10
RAID 10 combines mirroring and striping to create a redundant, high-performance RAID level. Data is striped across disks for performance, and each stripe is mirrored for redundancy. RAID 10 requires at least four disks but provides fast read/writes and can survive multiple disk failures.
Choosing software vs hardware RAID
There are several factors to consider when deciding between software RAID and hardware RAID:
Performance needs
For most common workloads, software RAID provides adequate performance. But hardware RAID will provide faster performance for I/O intensive applications thanks to dedicated resources like onboard cache, faster processors and custom firmware.
Features required
Hardware RAID controllers offer more advanced management capabilities like instant hot swapping of drives, virtualization support, and snapshot/rollback. Software RAID has more basic feature sets.
Reliability requirements
Mission critical systems that need 24/7 uptime will benefit more from the redundancy and failover capabilities of enterprise-grade hardware RAID controllers.
Drive interface and disks
If you need to combine disks of different interfaces or capacities, software RAID offers more flexibility. Hardware RAID works best with matching disks of the same interface.
Cost
Hardware RAID controllers can be very expensive, sometimes even more than the disks themselves. Software RAID has little to no cost and uses existing OS resources.
Ease of management
Hardware RAID management interfaces and tools provided by the vendor can offer easier setup and monitoring versus manual software RAID configuration. But software RAID gives more granular control.
Boot support needs
Hardware RAID arrays are more easily configured as boot volumes. Booting from software RAID can require more specialized configuration.
Software RAID vs hardware RAID comparison
| Factor | Software RAID | Hardware RAID |
|---|---|---|
| Performance | Moderate performance, depends on system CPU and resources. | Faster performance due to dedicated RAID processor and cache. |
| Features | Basic RAID levels and fault tolerance. | Advanced features like snapshots, instant hot swap, virtualization support. |
| Reliability | Provides fault tolerance but susceptible to OS and system crashes. | Enterprise-level redundancy and failover. |
| Flexibility | Can combine disks of different types, vendors, interfaces. | Requires same type of disks, less flexible. |
| Scalability | Easy to expand capacity by adding disks. | Limited scalability, may require identical controllers. |
| Cost | No cost, uses existing system resources. | Hardware RAID controllers are an added cost. |
| Complexity | Manual configuration more complex. | Hardware RAID tools simplify setup and management. |
| Boot support | Limited boot support depending on OS and configuration. | Full boot support standard on most controllers. |
| Cache support | No cache, depends on system RAM. | Onboard cache improves performance. |
As this comparison shows, hardware RAID excels at performance, reliability and features while software RAID offers better flexibility, scalability and cost-efficiency.
Software RAID vs hardware RAID: which should you choose?
Here are some guidelines on when to choose software vs hardware RAID:
Software RAID is a good choice when:
- You need basic RAID functionality with good fault tolerance.
- Cost savings is important and you have limited budget.
- You want flexibility to combine different drive types in a single array.
- Easy capacity expansion is required.
- High-end performance and advanced features are not needed.
Hardware RAID is a better choice for:
- Mission critical systems requiring maximum uptime.
- Performance sensitive applications like database or web servers.
- Enterprise environments needing advanced management capabilities.
- Data centers using fiber-channel SAN storage.
- Complex workloads that need dedicated resources and processing power.
If your needs fall somewhere in the middle, you may want to consider an alternative like hypervisor RAID or motherboard RAID.
Hypervisor RAID
Hypervisor RAID allows guest virtual machines to access a RAID array managed at the hypervisor level. This removes the hardware requirement of RAID cards while still providing some performance and management advantages.
Motherboard RAID
Some motherboards have built-in RAID support through onboard RAID controllers. This gives some benefits of hardware RAID without requiring a separate card. However, performance is often still lower than dedicated RAID controllers.
Conclusion
Software RAID and hardware RAID both provide benefits like enhanced performance and fault tolerance. But software RAID offers more flexibility and lower cost, while hardware RAID provides higher performance and advanced enterprise features.
For home users and small businesses, software RAID is usually sufficient and the better choice. But for demanding 24/7 environments, hardware RAID is preferable. By understanding the differences between the two implementations, you can select the right RAID solution to match your needs and budget.