All-Flash Right Amount
Using the appropriate amount of flash for caching high-performance disk drives can benefit more than adding 10,000 or 15,000 RPM disk drives at the problem. While the disks certainly will be less expensive than the flash, one could end up wasting a lot of disk capacity and still not gain the performance benefits of flash, eliminating the cost benefit of the disk.
Marketing vice president at Load DynamiX, recommends that companies create workload models before making any changes to their storage infrastructure. He agrees with Flisher that once the company establishes a need for flash, the next step is determining what kind of flash is needed, such as read- or write-optimized. Ideally, he says, before a company invests large amounts of money on buying flash arrays to front-end its disk storage it should test its workload model with differing amounts of flash in order to determine the best mix of flash to disk.
When simulating these performance tests, he says, the company should be able to determine at what point the targeted response time falls to unacceptable levels. By modifying the amount of flash in the simulations, the company should be able to determine not only their immediate needs, but how much additional flash is needed as the company grows.
For example, if a simulation shows that having 5 percent flash meets the immediate performance needs but 7 percent will meet the needs for the company to grow a predetermined amount, such as 20 percent, then perhaps the company should opt for the 7 percent flash array to build in scalability.
Unlike disk drives that perform at essentially the same levels when the disk is new and when it's been used for a while, flash needs to be "preconditioned," Rosenthal says. As flash is used, it builds up to a steady-state environment, he says, which can be considerably faster than its initial state.
It is essential for the IT manager to know up front what software services the vendor built into their flash arrays. Some software services, such as deduplication and compression, are built in, for example, while other vendors offer these services as options. Before a company purchases a flash array, Rosenthal says, the IT manager should inquire as to whether these services are on all the time or not. Having the services turned on automatically could result in lower performance. In applications where performance is the primary reason for moving to flash, such as in a transaction-heavy environment, he says, the company might opt for the services to be offered only as options.
Sometimes, however, performance hits might be acceptable. In an environment where flash is able to improve a storage subsystem's performance by 5 fold, a 50 percent performance hit, which only improves performance by 25 percent, might still be acceptable, he says.