What is an example of a software based RAID?

RAID (Redundant Array of Independent Disks) is a technology that allows combining multiple physical disk drives into one or more logical units to increase capacity, performance, and/or reliability compared to a single drive. There are two main types of RAID implementations – hardware RAID and software RAID. Hardware RAID uses a dedicated RAID controller, while software RAID relies on the operating system’s software and drivers to implement RAID functionality.

In software RAID, the processing necessary to implement RAID functionality such as striping (RAID 0), mirroring (RAID 1), parity (RAID 5), etc. is handled by the operating system and drivers. This removes the need for a dedicated hardware RAID controller. Software RAID can be implemented on standard computer hardware without any additional RAID hardware.

Some examples of software RAID solutions include:

Windows Dynamic Disks
Linux MD RAID
Apple RAID

ZFS and Btrfs file systems
Storage Spaces in Windows
Various third-party software RAID solutions

In this article, we will take a closer look at software RAID – what it is, how it works, pros and cons compared to hardware RAID, and provide an example setup and usage of Linux MD RAID.

Table of Contents

What is Software RAID?

Software RAID refers to implementing RAID functionality like striping, mirroring, parity, etc. through the operating system and software drivers rather than dedicated hardware RAID controller. The processing necessary for RAID is handled by the CPU and software.

With software RAID, standard off-the-shelf disk drives and hardware can be used to create RAID arrays. No special or expensive RAID controller card is required. The RAID management and configuration is done through the operating system itself.

Software RAID provides an economical, flexible alternative to hardware RAID and allows using the advanced features of RAID even with regular desktop and server systems without RAID cards.

Software RAID Benefits

Some benefits of using software RAID include:

Lower cost – No need for expensive hardware RAID cards.

Flexibility – RAID arrays can be created from regular disks on standard hardware.
Platform independence – Software RAID is OS and hardware agnostic.
Ease of management – RAID can be configured and managed through OS tools.

Portability – RAID arrays created with software RAID can be migrated to different systems.
Scalability – Additional disks can be added to array as needed.

Software RAID Drawbacks

Some potential downsides or disadvantages of software RAID include:

Increased CPU utilization – RAID processing loads the system CPU.
No dedicated cache – Uses system RAM rather than dedicated RAID controller cache.
Lack of onboard hardware RAID processor – Benefits like processor assisted parity calculation are not present.

Less efficient than hardware RAID – Software RAID has lower throughput compared to dedicated RAID cards.

Overall, while software RAID does not provide the highest performance, it offers a good balance of cost, flexibility, and features for many applications and usage scenarios.

How Does Software RAID Work?

Software RAID works by using the system CPU and RAM along with OS and device drivers to implement RAID functionality. Here is a high level overview of how software RAID works:

The individual physical disks that will be part of the RAID array are connected to the standard SATA, SAS ports on the system.
Partitions are created on each disk drive.
The software RAID solution will combine and manage the partitions on the member disks as per the desired RAID level (RAID 0, 1, 5 etc).

The RAID metadata and configuration data is stored in the reserved space of the member disk partitions or drives.
The OS loads the required device drivers and software RAID modules/services on boot up.
The device driver exposes the RAID arrays to the OS as regular block devices that can be formatted and mounted.

I/O requests to the RAID block device are intercepted by the software RAID drivers.
The software RAID logic determines how to service the I/O based on the RAID level configuration for that array. This may involve striping, mirroring, parity calculation etc.
The I/O request is split and directed to the member disks as per the RAID logic requirements.

The results are combined and returned back to the OS/requesting application.

This is a simplified overview of the key steps. The actual internal logic can be quite complex depending on the RAID level and the specific software implementation. However, the important point is that a dedicated hardware RAID card is not required, and the CPU/RAM/software is able to provide RAID services.

Linux MD Software RAID

Linux MD RAID (Multiple Devices) is a native software RAID implementation in Linux. It allows creating RAID arrays under Linux without any additional hardware.

Linux MD RAID supports RAID levels 0, 1, 4, 5, 6, 10 and other configurations. Multiple RAID arrays can be created, managed, monitored within Linux using common CLI tools and utilities.

Here is an overview example of creating a 2 disk RAID 1 array using Linux MD software RAID:

Example Software RAID Setup

System Hardware:

Linux server with 2 x 1TB SATA hard drives installed
No hardware RAID card present

Software:

Linux distribution with MD RAID support in kernel (all modern distros)

Objective:

Create a 1TB RAID 1 array using the 2 disks

Assume partition and filesystem creation is already done on both disks with partitions aligned

Steps:

Check current disk partitions:
```
# fdisk -l
```
This will list the current partitions on the two disks that will form the RAID 1 array. Assume they are /dev/sda1 and /dev/sdb1
Create the RAID1 array:
```
# mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sda1 /dev/sdb1
```
This will create /dev/md0 as a RAID1 array using the two partitions

Check RAID array status:
```
# mdadm --detail /dev/md0
```
This will show details on the new array – RAID level, member devices, status etc.
Format RAID array with filesystem:
```
# mkfs.ext4 /dev/md0
```
Format the RAID device with ext4 or any other filesystem
Mount RAID array to access storage:
```
 
# mount /dev/md0 /mnt/raid1
```
Mount the formatted RAID device on a mount point to start using the storage

The Linux mdadm utility can be used to monitor, manage, and perform other admin tasks on the software RAID array like adding disks, checking status, rebuilding, etc.

This demonstrates how Linux MD software RAID can be implemented without any special RAID hardware. The RAID 1 array provides mirrored storage for enhanced redundancy and protection against disk failures.

Software vs Hardware RAID Comparison

While software RAID provides an inexpensive, flexible RAID solution, dedicated hardware RAID has some advantages in performance and capabilities.

Here is a comparison between some pros and cons of software vs hardware RAID:

Software RAID	Hardware RAID
Pros: Lower cost as uses existing hardware Hardware agnostic – can recreate arrays on different hardware Easy to scale capacity by adding disks Managed through OS tools – flexible and portable	Pros: Better performance – dedicated RAID processor Additional cache memory improves speeds More mature and advanced implementations Better suited for mission critical deployments
Cons: Higher CPU utilization Slower performance compared to hardware RAID Limited to standard system hardware resources	Cons: More expensive due to RAID cards Less flexible – tied to specific hardware Upgrades typically require new RAID card

Software RAID

Hardware RAID

Pros:

Lower cost as uses existing hardware
Hardware agnostic – can recreate arrays on different hardware
Easy to scale capacity by adding disks
Managed through OS tools – flexible and portable

Pros:

Better performance – dedicated RAID processor
Additional cache memory improves speeds
More mature and advanced implementations
Better suited for mission critical deployments

Cons:

Higher CPU utilization
Slower performance compared to hardware RAID
Limited to standard system hardware resources

Cons:

More expensive due to RAID cards
Less flexible – tied to specific hardware
Upgrades typically require new RAID card

As can be seen, software RAID provides a good balance of cost, flexibility, and functionality for less demanding environments. Hardware RAID delivers superior performance and advanced management capabilities ideal for mission critical systems and workloads.

The choice between software vs hardware RAID depends on the specific application requirements, budget, and use case priorities for a given storage deployment scenario.

Conclusion

Software RAID allows implementing RAID functionality like striping, mirroring, parity etc. using standard hardware and the operating system software. It provides an inexpensive and flexible RAID solution without requiring a special hardware RAID card.

Some examples of software RAID include Windows Dynamic Disks, Linux MD RAID, Apple RAID, ZFS, and others. These leverage the CPU and system memory along with software drivers to handle the RAID operations.

While software RAID has some disadvantages compared to dedicated hardware RAID controllers, it offers a compelling set of benefits as well for many usage scenarios in terms of cost, flexibility, ease of use and scalability.

Linux MD RAID provides a native software RAID implementation in Linux. As seen in the example, it enables creating mirrored or striped arrays purely through software utilities like mdadm. Features like snapshots, spare disks, consistency checks etc. can provide a fairly robust software RAID solution.

Software RAID opens up the capabilities of RAID technology even on regular systems without expensive RAID cards. With its advantages and use cases, software RAID has become widely popular among home users, small offices, media centers, and non-critical workloads.