Mixing Flash, HDD Storage to Cater to Changing Workload Needs

by Chris Preimesberger

For sequential writes, disk outshines solid-state drives (SSD), delivering up to 40MB per second with a single 7.2K rpm drive. Ten hard disks equal the sequential throughput of an SSD device at a fraction of the cost. That makes it possible to get a lot more performance from a disk, while leveraging its capacity advantages.

NAND flash is based on solid-state technology. Since flash eliminates mechanical parts, access times are as much as 100 times faster than with a hard-disk drive.

The working set size (the actual amount of application data that needs to reside on flash) for low sub-millisecond responsiveness is no greater than 10 percent, and that includes the most performance-intensive applications. So most companies don’t need 100 percent flash for predictable read performance.

Failures on disk are randomly distributed, so the probability of multiple drive failure is low. Over time, multiple SSDs simultaneously could be at the end of their lives, with much greater risk of multiple disk failures leading to data loss.

Flash storage systems—like any other type of storage systems—have to accommodate for RAID and snapshot data. However, this data takes up precious SSD space, reducing the usable capacity available for application workloads. Factor in the costs associated with overprovisioning flash to prolong overall endurance, and going the all-flash route for all applications can be expensive.

Almost all estimates point to the price of flash dropping 20 percent per year, but the endurance of multi-level cell (MLC) flash will decline along with it; any cost savings will be negated by having to overprovision to compensate for lower endurance/faster burnout.

While it appears simple to scale performance by adding more disk drives, you can only go so far before you’ve got a mountain of unused capacity—and a hefty power and cooling bill.

The winning storage architecture will allow customers to use the right mix of media (flash and disk) to cater to their diverse and evolving workload requirements.