What is RAID?
RAID stands for Redundant Array of Independent Disks. It is a data storage technology that combines multiple disk drive components into a logical unit. RAID provides increased storage performance, reliability, fault tolerance and data redundancy (Cambridge Dictionary, 2022).
The main goals of RAID are to provide improved performance, protection against drive failures, and increased capacity. Some key benefits of RAID include increased read/write speeds, ability to reconstruct data after a drive failure, and the ability to combine multiple drives into larger logical volumes (Merriam-Webster, 2022).
There are several standard RAID levels, each with its own mix of performance, redundancy, and capacity:
- RAID 0 – Data is striped across drives for performance, but offers no redundancy.
- RAID 1 – Drives are mirrored for redundancy, but usable capacity is halved.
- RAID 5 – Data is striped with distributed parity for redundancy while retaining capacity.
- RAID 6 – Similar to RAID 5, but with dual distributed parity.
- RAID 10 – Combines mirroring and striping for both performance and redundancy.
Overall, RAID aims to provide improved reliability and performance compared to single drives, by combining multiple drives in a redundancy scheme (Dictionary.com, 2022).
What are SSDs?
SSDs, or solid-state drives, are a type of non-volatile computer storage media that stores persistent data on solid-state flash memory (source). Unlike traditional hard disk drives (HDDs) that store data on spinning platters, SSDs use microchips and contain no moving parts. This allows SSDs to access data much faster than HDDs.
Some key benefits of SSDs compared to HDDs include:
- Faster read/write speeds – SSDs can read and write data very quickly, often 10x faster than HDDs. This makes them better suited for tasks that require fast access.
- Better durability – Since SSDs have no moving parts, they tend to be more resistant to shock and vibration.
- Lower latency – SSDs can access data almost instantly, while HDDs require time for the platter to spin and the head to move.
- Lower power consumption – SSDs consume less power than HDDs, which is advantageous in mobile devices.
The main downsides of SSDs are higher costs per GB compared to HDDs, and limited capacities on some models. However, prices have been dropping while capacities continue to increase, making SSDs a viable option for more and more applications.
Mixing HDDs and SSDs in RAID
While it is technically possible to mix HDDs and SSDs in the same RAID array, this configuration comes with tradeoffs and is not recommended in most cases. The main considerations around mixing drive types in RAID include:
Overview:
- Performance – Mixing HDDs and SSDs can lead to uneven performance based on how data is distributed across the drives. The SSDs will be much faster than the HDDs, but the RAID as a whole is limited by the slowest drive.
- Reliability – SSDs and HDDs have different failure rates and lifespans. This can impact the array rebuild time and likelihood of failure.
- Compatibility – The RAID controller and firmware must support mixing drive types. Some controllers do not allow this configuration.
Pros:
- Cost savings – Adding a small number of SSDs to a primarily HDD array can provide a performance boost for a lower cost than using all SSDs.
- Caching benefits – The SSDs can act as a read/write cache to boost performance for certain workloads.
Cons:
- Uneven performance – As mentioned above, mixing drive types can lead to uneven performance across the array.
- Increased failure risk – If an SSD fails, the rebuild time will be much longer compared to replacing another HDD. This exposes the array to greater risk of a second drive failure during rebuild.
- Wasted SSD capacity – With some RAID levels like RAID 5/6, the SSD capacity may not contribute fully to the overall array capacity.
Overall, while possible, mixing HDDs and SSDs in a single RAID array introduces caveats and tradeoffs. Most experts recommend maintaining separate arrays by drive type or using enterprise SSDs specifically designed for mixed workload environments.
Performance Considerations
When it comes to speed, SSDs significantly outperform HDDs. An average HDD has read/write speeds between 80-160MB/s, while SSDs can reach 550MB/s or higher for reads and 500MB/s for writes (Salvagedata.com). This massive difference in throughput allows SSDs to provide much faster access times, usually under 0.1ms compared to HDDs which are typically between 2-5ms (Stellarinfo.com).
This performance advantage is maintained when using SSDs in a RAID configuration. Tests show RAID 0 SSD arrays can achieve over 1GB/s read/write speeds. In comparison, even large RAID 0 HDD arrays top out between 200-400MB/s. The dramatically faster access times of SSDs also benefit other RAID modes like RAID 10 that optimize for redundancy and read performance (Salvagedata.com).
For uses like gaming, virtualization, video editing or as boot drives, SSD RAID arrays provide a substantial speed boost over HDD RAIDs. The only downside is the limited capacity of SSDs. For mass storage of media or backups, HDD RAID still offers better value (Tomshardware Forum).
Reliability Factors
SSDs tend to be more reliable than HDDs for most consumer use cases according to recent research. SSDs have no moving parts and utilize wear leveling techniques to distribute writes across all cells of the drive evenly. This helps minimize cell degradation over time. One study by Backblaze examining over 100,000 HDDs and SSDs found SSDs had an annual failure rate of 1.2% versus 1.8% for HDDs (https://www.backblaze.com/blog/how-reliable-are-ssds/).
However, HDDs can potentially last longer if lightly used. HDD failure rates increase sharply once drives exceed 3 years of service life. SSDs do not display this same age-related failure pattern, but their cells have a finite number of write cycles before wearing out. With heavy writes, SSDs may wear out in 3-5 years, while lightly used SSDs can often exceed 10 years of life. Ultimately SSDs are generally recommended for most consumer use cases like laptops and desktops due to their speed, silence, and resistance to shocks. But for archival storage and backups that see less frequent writes, HDDs can potentially prove more cost effective.
RAID Controller Support
When mixing HDDs and SSDs in a RAID configuration, it’s important to have a RAID controller that fully supports both drive types. Many older RAID controllers were designed primarily for HDDs and may not work optimally with SSDs.
Key factors to consider for RAID controller support include:
- Driver support for SSDs – Make sure the controller has up-to-date drivers that enable all SSD features like TRIM.
- Caching algorithms – The controller should have caching optimized for SSD performance and endurance.
- Hot spares – The ability to designate SSDs as hot spares for HDDs (and vice versa) provides more flexibility.
- Tiered storage support – Being able to combine SSDs and HDDs in an automated tiered storage configuration is ideal.
Overall, look for enterprise-level RAID controllers that specifically advertise SSD support and compatibility. For example, the Intel RAID Controllers allow mixing drive types and include optimizations like write coalescing for SSDs.
On the software side, some operating systems like Windows Server and many Linux distributions have built-in RAID that may need driver or firmware updates to fully support SSDs in mixed configurations.
Ideal RAID Configurations
When mixing SSDs and HDDs in a RAID array, best practices recommend assigning the SSDs to dedicated RAID groups for optimal performance. The HDDs can be configured in their own RAID group(s) for capacity.
For example, configuring two SSDs in RAID 1 for redundancy while getting fast read speeds from both drives. The HDDs could then be placed in a separate RAID 5 or RAID 6 array to provide larger capacity storage and protection against drive failures.
Popular configurations for mixed SSD/HDD RAIDs include:
- RAID 1 SSD mirror for OS and applications, RAID 5/6 HDD array for data storage
- RAID 10 SSD array for I/O performance, RAID 6 HDD array for capacity
- Tiered storage with RAID 1 SSDs for hot data, RAID 5 HDDs for cold/archival data
The key is to maximize the strengths of each storage media – SSDs for speed and HDDs for capacity. Keeping them in separate RAID groups allows you to do that effectively in a mixed environment.
Tiered Storage
Tiered storage is an architecture that utilizes different types of storage media to optimize cost and performance. With tiered storage, frequently accessed “hot” data is stored on faster media like SSDs, while less frequently accessed “cold” data is stored on slower, high capacity media like HDDs.
SSDs provide significant performance benefits for tiered storage implementations due to their fast read/write speeds compared to HDDs. By using SSDs as a cache or high performance tier, organizations can accelerate applications and workloads that require frequent access to storage. According to TechTarget, tiered storage with SSD caching can deliver “100 times greater performance than what is possible with hard drive arrays.”1
When implementing SSD tiering, administrators must carefully consider which data will benefit most from the SSD tier. Placing frequently accessed data like indexes, metadata, and temporary files on the SSD tier can maximize performance gains. The high cost of SSDs compared to HDDs makes selective tiering important for optimizing value. Overall, tiered storage with SSDs offers a flexible and cost-effective architecture for balancing performance and capacity requirements.
Cost Considerations
The cost of SSDs versus HDDs is one of the main factors when considering mixing drive types in a RAID configuration. SSDs currently have a higher cost per gigabyte compared to HDDs. For example, according to Avast, a 1 TB HDD costs around $60 while a 1 TB SSD can cost over $100. The price difference is due to the more complex hardware and architecture required for SSDs.
However, SSD prices have been steadily declining over the years while HDD prices have remained relatively flat. According to a price comparison chart on Reddit, in 2013 the lowest cost per TB for SSDs was around $625 while HDDs were around $60 per TB. In 2023, SSDs can be found for under $100 per TB while HDDs remain around $60 per TB.
For RAID configurations, the higher cost of SSDs can be prohibitive for large storage arrays consisting solely of SSDs. A hybrid RAID configuration with a smaller number of SSDs and larger number of HDDs can provide a cost-effective solution.
Summary
To summarize, mixing SSDs and HDDs in a RAID setup is possible but needs to be approached with care and consideration.
The key factors to consider are:
- RAID level – Some levels like RAID 0 and 1 are better suited for SSD/HDD mixing than levels like RAID 5 or 6.
- Performance – SSDs are much faster than HDDs, so an imbalance can create bottlenecks. Tiered storage helps optimize for cost and performance.
- Reliability – SSDs can have shorter lifespans than HDDs in terms of drive writes. Make sure RAID level accounts for this.
- Compatibility – The RAID controller needs to support SSDs and handle TRIM commands appropriately.
The best practices are to use RAID 0/1 for performance, consider tiered storage to optimize cost and performance balance, ensure the RAID controller supports TRIM, and plan drive replacements carefully accounting for SSD lifespan.
By understanding the considerations around mixing drive types in RAID, you can take advantage of SSD speed while still utilizing cost-effective HDD capacity. With proper planning, an SSD/HDD RAID array can provide the ideal storage configuration.