The Storage Networking Industry Association – SNIA – determined a few years back that it should address SSDs since they were about to become an important part of most storage systems. To this end SNIA created the Solid State Storage Initiative, or SSSI.
They didn’t name it after SSDs since there will clearly come a time when flash stops pretending it’s an HDD and abandons standard HDD mechanical and interface specifications.
Two important efforts of the SSSI are to educate potential users about the benefits of using SSDs, and to develop a standard means of testing SSDs so that they can be accurately compared against one another. The latter is very important since nearly every SSD maker specifies a product’s performance according to the manufacturer’s own set of rules. Most manufacturers specify the best possible performance, which occurs when the SSD is fresh and new, and has no need to manage thorny problems relating to stale data.
After about a year of discussions and negotiations, the SSD makers and users in SSSI agreed to the SNIA Performance Test Specification, or PTS.
The PTS tests SSDs in three phases:
- First the SSD is erased to bring it to a “like new” state
- The SSD is then overwritten twice with random data to ensure that every flash block has been touched.
- Lastly, the actual test sequence is run repeatedly until the SSD reaches a “Steady State” in which the average latency for a round of testing does not change more than 20% for five rounds, and remains within 5% of a line with a 10% slope in this window. At this point its performance can be measured and reported.
Since there are many potential usage models, this last step is tailored to measure the SSDs’ performance under a workload similar to that of a specific system.
The SNIA SSSI PTS specification can be downloaded for free at http://www.SNIA.org/pts.
An Objective Analysis report showing the results of tests on sixteen SSDs and one HDD can be purchased by clicking HERE. This report, authored in conjunction with Coughlin Associates, is based on data measured by Calypso Systems.