A software RAID controller is a type of RAID controller that uses the system’s CPU and software to implement RAID functionality rather than using dedicated hardware. Software RAID does not require any additional hardware beyond the standard components already present in a computer system. The software RAID algorithms are implemented at the operating system level or via dedicated software applications.
What are the benefits of software RAID?
There are several potential benefits to using software RAID controllers rather than hardware controllers:
- Cost – Software RAID does not require purchasing any additional hardware, so it can be a lower cost solution compared to hardware RAID.
- Flexibility – Software RAID implementations allow more configuration options compared to hardware RAID. There are more available RAID levels and stripe sizes.
- Platform independence – Software RAID can be implemented across different platforms and operating systems using the same software, while hardware RAID is dependent on specific controller hardware.
- Scalability – With software RAID, increasing array capacity is as simple as adding more storage drives. Hardware RAID capacity depends on the controller hardware limitations.
What are the disadvantages of software RAID?
There are also some potential disadvantages to using software instead of hardware for RAID:
- Performance – Software RAID uses system resources like CPU, RAM, and bus bandwidth for RAID tasks. This can result in lower performance compared to hardware controllers with dedicated RAID processors.
- Reliability – If the operating system crashes, software RAID may be vulnerable to failure or data corruption. Hardware RAID controllers have their own dedicated memory to protect array data.
- No caching – Hardware RAID controllers usually have onboard caching that optimizes read/write performance. Software RAID configurations lack this caching ability.
- Administrative overhead – Managing software RAID requires loading the RAID management software. Hardware RAID configurations can be more seamlessly managed directly through the controller interface.
What are the differences between hardware and software RAID?
| Factor | Hardware RAID | Software RAID |
|---|---|---|
| Cost | Requires purchase of controller | No additional hardware needed |
| Performance | Dedicated processor optimized for RAID tasks | Relies on system CPU and resources |
| Reliability | Onboard memory provides data protection | Vulnerable to OS and system crashes |
| Flexibility | Limited RAID options dictated by hardware | Fully customizable options in software |
| Caching | Onboard cache improves read/write speeds | No caching available |
| Scalability | Constrained by hardware limitations | Scales easily by adding drives |
| Platform dependence | Dependent on hardware | Can be implemented across different platforms |
What are the different levels of RAID?
There are several standard RAID levels that provide different combinations of performance, redundancy, and capacity:
- RAID 0 – Striping without parity or mirroring. Provides fast performance but no redundancy. If one drive fails, all data is lost.
- RAID 1 – Mirroring without parity or striping. Provides redundancy by writing duplicate copies of data to paired drives.
- RAID 5 – Striping with distributed parity. Provides a balance of speed and redundancy. If one drive fails, data can be rebuilt from the parity information.
- RAID 6 – Striping with double distributed parity. Provides fault tolerance up to two failed drives, but write performance may be slower than RAID 5.
- RAID 10 – Striping and mirroring without parity. Provides speed and redundancy by combining RAID 0 striping and RAID 1 mirroring.
RAID 0
RAID 0 arrays stripe data across multiple drives without parity or mirroring. This provides fast read and write speeds by spreading contiguous data across multiple disks that can operate in parallel. However, RAID 0 provides no redundancy – if one drive fails, the entire array will be lost. RAID 0 is useful in scenarios where speed is more important than data protection.
RAID 1
RAID 1 arrays mirror data between pairs of member drives. When data is written, identical copies are stored on each drive simultaneously. This means that if one drive fails, the data is fully intact and available on the mirrored drive. RAID 1 provides redundancy and fault tolerance but at the cost of usable capacity, since two physical drives are required for each mirrored pair. Write performance can also be slower compared to other RAID levels.
RAID 5
RAID 5 stripes data across three or more member drives, while also writing parity information to a distributed disk in the array. If any single drive fails, the missing data can be recreated from the parity block and the remaining drives. RAID 5 requires at least three drives to implement. It provides a balance of speed, capacity, and redundancy, which makes it a popular general purpose RAID level.
RAID 6
RAID 6 is similar to RAID 5, but uses double distributed parity instead of single parity. The extra parity allows the array to withstand failures of up to two drives. If two drives fail simultaneously, the data can still be recreated from the parity blocks on the remaining drives. However, the double parity makes write operations slower compared to RAID 5. RAID 6 requires a minimum of four drives.
RAID 10
RAID 10 combines both mirroring and striping. It creates mirrored pairs of drives, then stripes data across each set of mirrored drives. This provides redundancy through mirroring while also enhancing read/write performance via striping. RAID 10 requires an even number of drives, with a minimum of 4. It provides fast performance and good fault tolerance but at the expense of 50% of the total capacity for redundancy.
What are some examples of software used for software RAID?
There are a number of software solutions available for implementing software RAID on various operating systems and platforms:
- Windows – Windows software RAID can be implemented using Disk Management or Windows Storage Spaces.
- Linux – The Linux md driver allows creating software RAID arrays. Common tools for management include mdadm and LVM.
- macOS – SoftRAID is a commercial software RAID solution for macOS.
- Storage Spaces – Microsoft’s built-in software RAID for Windows and Windows Server.
- ZFS – The ZFS file system from Oracle Solaris implements RAID-like functionality.
- UnRAID – Software RAID designed specifically for media storage and redundancy.
- FreeNAS – Free storage OS based on FreeBSD that supports software RAID.
How does software RAID work?
Software RAID is implemented by the operating system and software drivers at the logical volume level. Here are the general steps for how software RAID works:
- RAID software drivers are loaded into the kernel and operating system.
- Physical disks are designated as RAID array members and partitioned.
- The RAID configuration utility initiates and formats the RAID partitions on the drives.
- The RAID logic distributes data across the array based on the parameters for that RAID level.
- The array volume appears to the OS as a single logical drive.
- Reads and writes go through the RAID software, which handles distributing data across drives.
- If a physical drive fails, the RAID software manages rebuilding the data from parity.
Software RAID handles all the RAID virtualization duties instead of dedicated hardware. The RAID functionality is transparent to the operating system and applications.
What role does the system CPU play in software RAID?
With software RAID, the system CPU takes on the computational work that a hardware RAID controller would normally handle. Key tasks include:
- Running the RAID software calculations and algorithms
- Managing the distribution of data across member drives
- Performing read/write operations across RAID array members
- Calculating and writing parity data
- Monitoring and rebuilding failed drives
- Scrubbing arrays to check data integrity
These additional RAID duties compete for CPU resources alongside normal system and application tasks. Insufficient CPU power can lead to slower RAID performance. High end CPUs or additional cores help mitigate this downside.
What RAID levels are best for software RAID implementations?
Some RAID levels are better suited for software RAID implementations from a performance perspective:
- RAID 0 – RAID 0 stripes efficiently in software, providing fast performance for large sequential reads and writes.
- RAID 1 – Mirroring places minimal computational load for writing copies to multiple drives.
- RAID 10 – Combining mirroring and striping can offer faster speeds via striping across mirrored drives.
- RAID 5 – Parity calculations can consume more CPU resources. Performance depends on CPU power.
- RAID 6 – Double parity increases CPU load. RAID 6 performs better with faster processors.
In general, simpler RAID levels like 0, 1, and 10 perform better in software compared to more computationally intensive parity levels like 5 and 6.
How does drive interface (SATA, SAS, NVMe) impact software RAID performance?
The drive interface used for member disks in a software RAID array can significantly impact overall performance:
- SATA – SATA 3Gb/s and 6Gb/s are the most common drive interfaces used for software RAID. But SATA speeds are limited compared to other interfaces.
- SAS – SAS drives provide higher bandwidth than SATA, improving performance for software RAID arrays.
- NVMe – NVMe SSD drives offer very high bandwidth through PCIe x4 or greater connections. This reduces interface bottleneck for software RAID.
Using SATA SSDs can remove the mechanical bottlenecks of hard drives. But even SATA SSD software RAID is limited by the SATA interface bandwidth. To maximize performance, choosing disks that utilize NVMe or SAS interfaces helps software RAID reach full potential.
How does system memory impact software RAID performance?
Having adequate RAM is important for optimal software RAID performance. Key factors include:
- Memory capacity – Larger arrays require more memory for caching and RAID computations.
- Memory speed – Faster memory improves access speeds for cached array data.
- Parity levels – RAID 5/6 require memory to buffer writes during parity calculations.
Insufficient RAM capacity can force paging of RAID data to disk which drastically slows operations. Using faster memory with adequate capacity prevents paging and helps software RAID achieve much higher performance.
Can you use hardware and software RAID together?
It is possible to combine both hardware and software RAID together in some scenarios, such as:
- Using hardware RAID for internal storage and software RAID for external storage arrays.
- Combining multiple hardware RAID arrays into a larger array via software RAID.
- Creating nested RAID levels, such as RAID 10 arrays on top of RAID 5 arrays.
- Using hardware RAID for redundancy and software RAID for added performance.
Any combination of hardware and software RAID must ensure that the layers of abstraction don’t create unexpected performance issues. But built correctly, complementing hardware RAID with software RAID capabilities can provide performance, capacity, and redundancy not available from a single solution.
Can you convert between software RAID and hardware RAID?
Converting between software RAID to hardware RAID and vice versa is possible in some cases but not an easy or straightforward process. Key considerations include:
- Data preservation – The existing data must be read from one system and written to the new system.
- Volume sizes – Capacity differences may require resizing arrays.
- RAID levels – Any differences between RAID levels must be taken into account.
- Filesystems – The filesystem may need to be converted to be compatible.
- Partition alignment – Conversion may impact partition alignment and performance.
Migrating all data intact requires copying the full contents over to the new system. Downtime is required during the migration. Due to the complexity, rebuilding the array from scratch on the new system is often a better approach than converting.
Conclusion
Software RAID provides a flexible, cost-effective alternative to dedicated hardware RAID controllers. By leveraging the system CPU and software drivers, software RAID delivers comparable RAID functionality without specialized hardware. RAID levels like 0, 1, 5, and 10 can all be implemented in software. Performance is heavily influenced by factors like CPU performance, system memory, and drive interfaces. Software RAID simplifies storage provisioning with most major operating systems and allows building arrays from commodity hardware. While lacking some of the reliability and cached performance of hardware RAID, software RAID solutions nevertheless empower users to configure robust, highly scalable storage with their existing system resources.