Decision makers looking to find the best solution for future server purchases have many reasons to relax. You will probably have to strictly consider the existing and projected performance requirements and work on the basis of worst-case scenarios. This means continuing on existing hardware, which should provide some time to carefully plan the next steps.
Although it will still take a few years, the future isn’t as bright for the hard drive anymore. Flash SSDs require a fraction of the power, and yet can deliver many times the I/O performance of conventional hard drives, or even hard drive arrays. I/O-intensive workloads are handled best by business-class flash-based SSDs if you can live with significantly smaller capacity points.
While SSDs still do not always provide reliable and especially predictable performance, future models will certainly be your top choice. A small array of a few flash SSDs will soon be perfectly capable of replacing an entire server rack, delivering more I/O performance, using much less power, and significantly reduced cooling cost. Take your time to plan thorough and to validate hardware carefully to exclude any issues SSDs might still have in stock today. Almost all SSD products available today are available in the 2.5” form factor. The only transitional product we've seen so far is OCZ's IBIS.
If you need lots of storage, then you have no option but to stay with 3.5” drives. Three terabyte capacity per drive was reached in the consumer space a few months ago and will soon be available for business and enterprise applications as well. Existing 3.5” SAS infrastructure is perfectly capable of working with these drives, although they are mostly based on SATA interfaces. You will even find power consumption and efficiency advantages, as 3.5” SATA, high-capacity drives are all based on 7200 RPM spindle speeds, requiring less cooling and delivering much increased capacity per space than 3.5” SAS drives.
This article caters to the mainstream SMB and entry-level enterprise segment, in which the step into flash SSDs is oftentimes not needed, too costly, or too early, and where robust solutions are most important. It is definitely possible to replace aged drives with modern products with no effort and minimal risk.