Solid state drives (SSDs) have become a popular storage option for both consumer and enterprise applications due to their fast performance and reliability. As SSD prices continue to drop, many are considering using SSDs in redundant array of independent disks (RAID) configurations to achieve improved performance, capacity, and data protection. However, SSDs have different characteristics than traditional hard disk drives (HDDs) that impact their effectiveness in RAID environments. In this article, we will examine the pros and cons of using SSDs for RAID and provide recommendations on how to configure SSD RAID arrays optimally.
How RAID Works
Before looking specifically at SSD RAID, let’s review how RAID works in general. RAID combines multiple physical drives into a single logical drive to provide increased storage capacity, speed, and redundancy for data protection. There are several standard RAID levels, each with different performance and fault tolerance tradeoffs:
– RAID 0 stripes data across multiple drives for faster performance but provides no redundancy. If one drive fails, all data is lost.
– RAID 1 mirrors data across two drives for redundancy. If one drive fails, data can be rebuilt from the other mirrored drive.
– RAID 5 stripes data across three or more drives and uses distributed parity to provide fault tolerance for one drive failure.
– RAID 6 provides fault tolerance for up to two drive failures by using dual distributed parity.
– RAID 10 combines mirroring and striping for both speed and redundancy. Data is mirrored and striped across four or more drives.
The RAID controller handles the distribution of data across the drives according to the RAID level. If a drive fails, the controller manages the rebuild process using the parity or mirrored data to reconstruct the lost data onto a replacement drive.
Benefits of SSD RAID
SSDs provide significant advantages over HDDs in key areas:
Faster Access Speeds
SSDs have much lower access latencies and faster random I/O performance compared to HDDs because they access data electronically with no moving parts. Average SSD latency is under 0.1 ms compared to HDD latency of 2-10 ms. This speed advantage makes SSD excellent for hosting operating systems, applications, or server workloads with busy random I/O activity.
Better Reliability
With no moving parts, SSDs have lower failure rates than HDDs. They are less prone to damage due to shock or vibration. SSDs still have a finite lifespan related to program/erase cycles, but modern SSDs can withstand thousands of cycles before wear becomes a concern.
Lower Power Consumption
SSDs consume much less power than HDDs, using just 5-10% of the power per gigabyte. This helps reduce energy costs for data centers and edge computing needs. The lower power draw also reduces heat output, which improves system cooling requirements.
Compact Design
SSDs come in much smaller physical sizes than HDDs for a given capacity. This compact, lightweight design makes them well suited for space-constrained server racks and devices. Multiple SSDs can fit into the same space as a single HDD.
By combining multiple SSDs into a RAID array, you can take advantage of these SSD benefits for increased capacity, speed, and redundancy. For applications with fast I/O requirements, SSD RAID can significantly outperform HDD RAID.
Considerations for SSD RAID
While the benefits are substantial, SSDs have some unique characteristics that require additional considerations when planning an SSD RAID implementation:
RAID Controller Support
SSDs use the NVMe (non-volatile memory express) protocol rather than the legacy SATA protocol used for HDDs. To take full advantage of SSD speed, you need an NVMe-capable RAID controller. Software RAID through the operating system can work, but dedicated hardware RAID controllers will provide the best performance.
TRIM Support
To maintain consistent write performance, SSDs need to periodically clean up invalid pages via the TRIM command. If TRIM is not enabled, write performance will degrade over time. Make sure your SSDs and RAID controller support TRIM for RAID arrays.
Spare Drives
Due to the risk of performance degradation over time, you may want to configure a hot spare SSD that can be rebuilt more frequently to maintain optimal write performance.
Drive Wear
For write-heavy workloads, SSDs are susceptible to wear as the drive nears its endurance limit on program/erase cycles. Wear leveling provided by the SSD controller helps distribute writes, but performance may decline as the drive exceeds its rated lifespan. For RAID 5/6, parity writes can accelerate wear. Higher endurance drives are needed for write-intensive RAID.
Rebuild Times
The faster speed of SSDs also means rebuilds will complete much quicker than with HDD RAID. Very short rebuilds increase the likelihood of an unrecoverable read error (URE) during reconstruction. Adjust RAID controller timeouts and use enterprise SSDs engineered for RAID.
Best Practices for SSD RAID
Given the above considerations, here are some best practices to follow when implementing SSD RAID:
– Choose enterprise-class SSDs designed for 24/7 operation at higher endurance. Consumer SSDs are not optimized for RAID environments.
– Use SSDs with power loss protection to avoid potential data loss or corruption during sudden power disruption.
– Select an enterprise RAID controller that supports NVMe SSDs, RAID levels 0/1/5/6/10, TRIM, and fast rebuild times. Avoid software RAID.
– Enable TRIM support on the operating system, SSDs, and RAID controller to maintain consistent write performance.
– Use RAID 5 instead of RAID 6 to reduce SSD wear from parity writes. With SSDs, rebuild times are fast enough to tolerate a single drive failure.
– Configure hot spare SSD(s) to allow proactive rebuilds before drives exceed wear thresholds.
– Monitor SSD wear levels and preemptively replace drives approaching end of life.
– Enable SSD patrol reads to proactively detect and repair data errors.
– Use RAID 1 mirroring for write-heavy workloads that will quickly wear out drives. The mirrored copy reduces wear.
– Consider overprovisioning spare area on the SSDs to allow the controller more flexibility with wear leveling and garbage collection.
Sample SSD RAID Configurations
Here are some examples of effective RAID configurations using SSDs:
4x SSD RAID 10 for High Performance
– 4 x 1TB NVMe Enterprise SSDs
– RAID 10 provides striping for speed and mirroring for fault tolerance
– 2TB total usable capacity
– High performance for transactional workloads like databases
8x SSD RAID 6 for Large Capacity
– 8 x 2TB NVMe Enterprise SSDs
– RAID 6 can tolerate up to 2 drive failures
– 12TB total usable capacity
– Provides capacity, redundancy on a budget
6x SSD RAID 50 for Mixed Workloads
– 6 x 1.5TB NVMe Enterprise SSDs
– RAID 50 is nesting RAID 5 arrays in a RAID 0 stripe
– Provides increased performance and capacity
– Good mixed workload option balancing capacity, performance, and redundancy
Conclusion
SSDs can deliver significant performance, reliability, and efficiency benefits when used for RAID storage. However, careful consideration must be given to SSD wear lifecycles, TRIM support, and rebuild times to tailor the optimal RAID configuration for your specific application workloads. With appropriate enterprise SSDs, RAID controller, and configuration best practices, businesses can build high-speed SSD RAID arrays that leverage the strengths of solid state storage for improved data protection and workload performance. Replacing legacy HDD RAID with SSD RAID can future-proof storage infrastructure to meet escalating demands for faster data access.