Flash Vs. RAM Solid State Disks

As major vendors ready for entry into the solid-state disk (SSD) market with Flash memory systems, don't count out the traditional RAM SSD. Even though RAM SSDs are more expensive per capacity, companies like Texas Memory Systems are seeing continued growth in RAM-based SSD systems. Why? RAM SSDs have two advantages: speed and reliability.In the SSD market, speed is king and for customers needing to squeeze every ounce of I/O out of their systems, RAM SSD is still the only way to go. The answer is in the numbers. For comparison, a typical mechanical hard disk drive does 4- to 5-millisecond reads and writes and can sustain about 150 to 300 random I/O's per second.

The typical Flash SSD completes reads in about 200 microseconds (0.2 milliseconds) and 100,000 random read I/O's per second; very impressive when compared with disk. In read-heavy applications, you will see a significant performance increase. Writes, however, are as high as 2 milliseconds and can sustain up to 25,000 random write I/O's per second. While you will still see a performance increase on writes with some Flash SSD vs. hard disks, they're most impressive from a read-performance perspective.

RAM SSD, on the other hand, is significantly faster at both read and write operations. It performs 15-microsecond (0.015 milliseconds) reads and writes and 400,000 random I/O's per second. Significant performance improvement can be seen on both types of operations. The challenge with RAM SSD is that you are dealing with smaller capacity -- 128 GB is typical, but smaller sizes aren't uncommon. You are looking for applications that have specific files that can be moved to the SSD; redo logs, undo segments, indices, and frequently accessed tables are great examples.

Flash SSD has another write-related issue; it can only handle so many. The typical range for Flash SSD is around 1 million to 5 million write cycles. For most applications, this is many years worth of writes. Most enterprise Flash SSDs are made up of multiple Flash modules. Having multiple Flash modules is essential to delivering maximum bandwidth and high availability through RAID protection. Flash SSDs aren't a good fit for latency-sensitive, write-intensive applications; for example, accelerating redo logs, undo segments, and enterprise messaging.

For many applications, Flash SSDs will offer significant and affordable performance increases, but when you need more performance or have legitimate concerns about a high-write application, RAM SSDs are the way to go.

George Crump is founder of Storage Switzerland, an analyst firm focused on the virtualization and storage marketplaces. It provides strategic consulting and analysis to storage users, suppliers, and integrators. An industry veteran of more than 25 years, Crump has held engineering and sales positions at various IT industry manufacturers and integrators. Prior to Storage Switzerland, he was CTO at one of the nation's largest integrators.