RAID (Redundant Array of Independent Disks) allows combining multiple physical disks into a single logical unit. This provides increased performance, capacity, and reliability compared to a single disk. RAID can be implemented via hardware (a dedicated RAID controller) or software. Hardware RAID has traditionally been viewed as superior to software RAID, but improvements in operating systems have blurred the lines.
What is RAID?
RAID combines multiple physical disks and logically groups them into a single unit. Data is distributed across the disks according to the RAID level used. Some key advantages of RAID include:
- Increased capacity – Disks are combined, so the total capacity equals the sum of capacities of each disk.
- Improved performance – Disk operations can be parallelized across multiple disks.
- Enhanced reliability – Redundant disks provide fault tolerance. If one disk fails, data can be rebuilt from the remaining disks.
There are several standard RAID levels, each with different performance, capacity, and fault tolerance tradeoffs. Some common levels include:
| RAID Level | Description |
|---|---|
| RAID 0 | Disk striping. Improves performance but provides no redundancy. |
| RAID 1 | Disk mirroring. Provides redundancy but capacity equals one disk. |
| RAID 5 | Striping with distributed parity. Provides redundancy while using most of total capacity. |
| RAID 6 | Like RAID 5 but with double distributed parity. Can withstand failure of two disks. |
Hardware vs Software RAID
RAID can be implemented in hardware or software:
- Hardware RAID – Uses a dedicated RAID controller. This is a physical card installed in the computer. It handles all RAID calculations and distribution.
- Software RAID – RAID is implemented by the operating system. The OS handles distributing data across disks.
Hardware RAID has traditionally been considered superior due to factors like:
- Better performance – The RAID card is purpose-built for RAID tasks. It offloads the work from the CPU.
- Operating system independence – Hardware RAID works independently of the OS, avoiding any software bottlenecks.
- Increased reliability – The RAID controller contains its own processor and cache memory. This isolates it from OS crashes.
However, software RAID has improved over the years. Key advantages include:
- Cost – Software RAID uses existing system resources and doesn’t require an additional hardware purchase.
- Flexibility – Software RAID can be configured without physical access to the server. Levels can be changed dynamically.
- Advanced functionality – Software RAID can leverage the operating system kernel for features like snapshots, caching, and scheduling.
For home users and small businesses, software RAID is likely sufficient, especially when using modern operating systems. For mission-critical enterprise environments, hardware RAID may provide better reliability and performance. But the lines have blurred as software RAID capabilities improve.
Benefits of Hardware RAID
Here are some of the key benefits provided by dedicated hardware RAID controllers:
1. Better Performance
Hardware RAID provides faster disk operations because the RAID card offloads the computational work from the main CPU. The controller has its own fast processor optimized for storage tasks. It also has its own cache memory, allowing it to return data to the computer more quickly. For workloads with heavy disk I/O, hardware RAID delivers faster reads and writes.
2. Operating System Independence
Hardware RAID operates independently from the computer’s OS and software. It provides reliable RAID functionality even if the OS experiences problems. If the system crashes, the RAID controller will continue operating. This makes hardware RAID better suited for mission-critical, high-uptime environments. The RAID card abstracts the underlying disks from the operating system.
3. Increased Reliability
Because it runs independently from system hardware and software, hardware RAID offers increased fault tolerance. The controller has its own dedicated processor and memory. This isolates it from crashes or power fluctuations that could impact the computer itself. Hardware RAID cards also contain capacitors and batteries that allow them to flush cached data to disks in the event of a power loss.
4. Ease of Migration
With hardware RAID, the OS views the array as a single logical volume. This makes migrating the disks to a new system much simpler. The volume can be moved between different operating systems or computers without needing to reformat or reconfigure. Software RAID volumes are generally OS-specific and may not be portable.
5. Advanced Functionality
Higher-end RAID controllers offer advanced features not found in software RAID. This includes capabilities like onboard security encryption, battery backups, caching algorithms, and support for management protocols like SNMP. Management interfaces allow monitoring and configuring the RAID card remotely.
Disadvantages of Hardware RAID
Hardware RAID controllers do have some downsides:
1. Cost
Hardware RAID cards carry an additional upfront cost over software RAID solutions included with most operating systems. Cards must be purchased separately based on the I/O connectivity and number of drives needed. This cost may not make sense for smaller deployments.
2. Lack of Flexibility
With software RAID, levels can often be changed on the fly, and new drives can be added without downtime. Hardware RAID is less flexible – changing configurations may require a full rebuild. New disks usually require powering down the server to insert the card.
3. Vendor Lock-in
Hardware RAID solutions are vendor-specific. If the controller fails, replacing it with a different brand may be difficult or impossible without rebuilding the volumes. With software RAID, drives are not tied to a particular vendor.
4. Limited Portability
While hardware RAID volumes are generally portable between systems from the same vendor, moving them to a different controller brand may be challenging. Software RAID defines open standards that allow disks to move between machines running the same OS.
When Does Hardware RAID Make Sense?
Here are some use cases where dedicated hardware RAID controllers are worth the additional investment over software RAID:
Transactional Databases
Databases like Oracle, SQL Server, MySQL, and PostgreSQL rely on very fast and reliable disk I/O. The bus-level performance and cache memory of hardware RAID delivers lower latencies needed by transactional systems. This justifies the cost for high-throughput databases.
Virtualized Environments
Hypervisors like VMware and Hyper-V require storage with fast IOPS to support many concurrent VMs. Hardware RAID with technologies like write-back caching and SSD caching helps meet the demands of heavily virtualized environments.
High-Availability Clusters
For HA clusters where maximum uptime is critical, hardware RAID provides added reliability. The RAID controller operates independently of server failures and transparently handles rebuilding drives.
Media Production/Rendering
Graphic design, HD video editing, and 3D rendering deal with massive files and require high sustained throughput. Hardware RAID delivers the performance needed for smooth workflows when working with very large assets.
Dedicated Backup Servers
On servers dedicated to mass data backups, hardware RAID can provide the speed, capacity, and redundancy required for reliable backups. Large arrays also benefit from management features included with most controllers.
When is Software RAID Sufficient?
In many cases today, software RAID provides enough benefits without the cost of hardware RAID. Some examples include:
Home Computers
For home desktop PCs or laptops, the included OS RAID capabilities work well enough for most users. The improved redundancy and capacity can be worthwhile using software RAID.
Small Business File Servers
Small businesses without stringent uptime requirements can rely on software RAID for file server storage and redundancy. Features like snapshots and remote management provide ample convenience.
Lightly Virtualized Servers
Servers running only a few low-intensity VMs don’t demand the highest disk I/O performance. Software RAID on modern server OSes can easily handle these lightweight workloads.
Backup Targets
Before being written to tape or cloud storage, backups are first staging on disk. Software RAID provides inexpensive capacity and redundancy for backup repositories.
Media Streaming Servers
Media servers for streaming audio or video can utilize software RAID, since large sequential reads are not limited by CPU bottlenecks. Redundancy is beneficial but sheer speed is less critical.
Software RAID vs Hardware RAID Summary
Here is a quick comparison between hardware and software RAID:
| Hardware RAID | Software RAID | |
|---|---|---|
| Performance | Very fast – dedicated controller optimized for storage | Slower – relies on system CPU and memory |
| Reliability | Very reliable – isolates disks from OS failures | Reliable – leverages stability of modern OSes |
| Cost | More expensive – requires purchasing a controller | Minimal cost – uses existing system resources |
| Flexibility | Less flexible – changes require rebooting server | More flexible – software allows easier changes |
| Portability | Controllers often vendor-locked but OS-independent | Generally OS-specific but not locked to vendor |
Conclusion
Hardware RAID delivers benefits in performance, reliability, and advanced features – but at increased cost. For mission-critical environments, hardware RAID may be justified, especially for transactional databases, virtualization, high-availability, and demanding media applications.
However, software RAID has narrowed the gap and now provides sufficient redundancy, capacity, and portability for many use cases, including home computers, SMB file servers, backups, streaming media, and lightweight virtualization. The choice depends on workload specifics and budget constraints.
In most cases, even large general-purpose file servers can operate fine on software RAID. Evaluate your specific storage requirements, performance needs, and budget to determine if dedicated hardware RAID controllers are worthwhile or if today’s software RAID solutions are sufficient for your needs.