For as long as SSD lags behind traditional disk in cost per gigabyte of capacity, it will be necessary to balance transactional performance requirements, capacity requirements, and cost against each other to determine the best fit for a given set of workloads.
This balance is a bit difficult to see at first glance, but some quick spreadsheet work can illustrate the balance as it relates to cost. I did this by comparing one enterprise vendor's street prices for SATA, 10K SAS, 15K SAS, and SLC SSD SAN arrays against their respective capacities and expected performance. In this instance, I found that cost per raw gigabyte of capacity ranges anywhere from around $1.50 for SATA to $28 for SSD. Likewise, cost per I/O per second ranges from $29 for SATA to $5 with SSD. In the comparison, SAS/Fibre Channel ends up somewhere in the middle of the capacity-versus-performance spectrum, though much closer to SATA than to SSD.
If you've properly tiered data sets with similar capacity and performance characteristics together, determining whether SSD is a good fit for any given tier is fairly straightforward.
The hybrid option
If past history is any guide, SSD will likely overtake traditional spinning disk as the most popular primary storage media in the long term. However, in the near term there will be an increasing number of hybridized SSD/disk solutions that will attempt to bridge the capacity-versus-performance gap that lies in between the two.
One great example of this is Oracle's Sun Open Storage NAS platform. For the past few years, Sun (and now Oracle) has offered this traditional disk storage platform with a write-biased SSD called a Logzilla that stores the system's write-intensive ZIL (ZFS Intent Log). Likewise, a read-biased SSD can act as an enormous extension of the controller's read cache. Though not yet common, this hybridized SSD and disk architecture can significantly magnify spinning disk performance without the high cost of a full SSD deployment.
Another type of hybridization is starting to happen at the level of the individual disk. Seagate recently announced the Momentus XT solid-state hybrid drive, which combines 4GB of SLC NAND memory and a traditional SATA hard disk in the same disk package. By using the SSD space to store the most-often-used data, the disk can appear to dramatically outperform a standard SATA disk while costing only marginally more. Although this product is targeted at the consumer space, you can expect this type of hybridization to find its way into some enterprise products as well.
Putting it all together
While SSDs have some amazing advantages over spinning disk, capacity and cost are still significant limiting factors to widespread adoption. Just remember to do the math to ensure that they're really the best choice for your workloads before you commit yourself to deploying them.
This article, "The right way to use SSDs in your storage environment," originally appeared at InfoWorld.com. Read more of Matt Prigge's Information Overload blog and follow the latest developments in data storage and information management at InfoWorld.com.