What is SSD RAID (solid-state drive RAID)?
SSD RAID (solid-state drive RAID) is a methodology commonly used to protect data by distributing redundant data blocks across multiple SSDs.
The phrase redundant array of inexpensive disks (RAID) -- later changed to redundant array of independent disks -- emerged in the late 1980s when mechanical hard disk drives (HDDs) were the primary storage media. The primary purposes for RAID were to improve performance and provide fault tolerance.
Technology vendors have since extended the concept of RAID to servers and storage systems that use higher-performance NAND flash-based SSDs. SSD RAID is primarily used to protect against data loss in the event of a drive failure.
Storage systems in general have moved on from applying RAID at the whole-drive level, and redundancy is now applied to data at a finer granularity. As with conventional HDD-based RAID, data can be divided at the block level and distributed across multiple SSDs in a variety of ways.
RAID concepts and levels
There are three key concepts in RAID:
- Mirroring writes data simultaneously to two separate drives.
- Striping splits data evenly across two or more drives.
- Parity passes raw binary data through an operation to calculate a binary result, also referred to as a parity block, used for redundancy and error correction.
Standard RAID levels in use with HDD- and SSD-based systems include the following:
- RAID 0 for simple striping.
- RAID 1 for simple or multimirroring.
- RAID 3 for byte-level striping, plus a single drive dedicated to storing parity information.
- RAID 4 for block-level striping with a parity drive.
- RAID 5 for block-level striping with distributed parity, which requires at least three drives.
- RAID 6 for block-level striping with a double distributed parity scheme.
Striping, with no redundancy or parity, is often used to increase performance. Striping with parity or double parity strengthens data protection and data recovery capabilities. With most RAID configurations, storing redundant data blocks enables the system to reconstruct the lost information if one or more drives fail.
HDD-based RAID vs. SSD-based RAID
One of the original purposes of HDD-based RAID was to increase performance. An operating system (OS) sees the hard drive as one logical storage unit, but because read/write operations are spread across multiple storage drives, input/output (I/O) could be aggregated and carried out simultaneously, thereby speeding up performance and increasing throughput.
Storage systems generally don't use RAID to pool SSDs for performance purposes. Flash-based SSDs inherently offer higher performance than HDDs, and they enable faster rebuilds in parity-based RAID. Rather than improve performance, vendors typically use SSD-based RAID to protect data integrity if an SSD drive fails.
Some flash array vendors have developed SSD RAID strategies they claim go beyond standard RAID and offer advantages, such as minimizing the performance impact of some types of RAID. Other reasons flash storage vendors consider changes or alternatives to standard RAID include the differences in the way HDDs and SSDs fail.
When an HDD fails, the entire drive is lost. With an SSD, only a part or parts of the drive may fail. As a result, some vendors have used customized approaches to RAID for protection against a drive failure.
Advantages of SSD RAID arrays vs. HDD RAID arrays
The advantages of SSD-based storage arrays over HDD-based arrays include the following:
- Reduced access time and superior I/O performance. Ideal SSD RAID performance requires the optimal combination of microprocessor, cache, software and hardware resources. When all these factors work together in the best possible way, an SSD RAID can significantly outperform an HDD-based RAID system of comparable storage capacity.
- Less power consumption. A typical SSD consumes less power than an HDD. When large numbers of drives are combined, the power savings of an SSD RAID array compared with an HDD RAID array can translate to lower long-term operating costs. In large data centers, the improved efficiency of SSDs compared with mechanical HDDs can also reduce the cooling cost through the use of simpler cooling systems and lower electric bills.
Limitations of SSD RAID
SSD RAID has drawbacks largely related to the storage media. SSDs carry a higher price per gigabyte compared to HDDs of comparable storage capacity. NAND flash-based drives are limited to a certain number of program/erase cycles before they wear out, become unreliable and require replacement.
Although the best SSDs have life span expectancies comparable to mechanical HDDs, the replacement cost for an SSD exceeds the replacement cost for an HDD of comparable data storage capacity. However, pricing for SSDs is expected to continue to decline over time and become more competitive with HDDs.
Mixing SSDs and HDDs on the same RAID array
In situations where data needs to be available quickly, SSD technology makes sense. By contrast, data that doesn't need to be as immediately accessible can be stored on HDD storage devices. This suggests that both SSD and HDD drives could be connected into the same RAID array.
RAID 1 with mirroring and RAID 10 with mirroring and striping are good choices as they can designate SSDs to mirror the HDDs. It's important to remember that slower HDDs restrict the array's speed, and the smallest capacity drive determines the overall capacity of the array. The RAID controller must be able to support both SSDs and HDDs.
The future of SSD RAID
Considering the speed and performance of SSD technology, along with the likely decline in pricing, the use of RAID storage might decline simply because of SSD capabilities. However, RAID's popularity is far-reaching, and it likely won't disappear anytime soon.
SSD RAID arrays are likely to supersede ones based on HDD RAID. SSD technologies using Peripheral Component Interconnect Express and non-volatile memory express encourage the development of RAID controllers that support PCIe and NVMe SSDs. The use of software-based RAID technology in support of SSDs is also likely to increase. Other future trends include the use of artificial intelligence to optimize RAID system performance and the use of encryption for SSD RAID arrays to protect data.
Learn more about software RAID, how it works, what its benefits are and how it differs from hardware RAID.
