Question #2: Which SSD is right for your workload?
In the 1st blog of our 20 questions on SSD series, we looked at whether or not your SSD was qualified by your OEM, opening the gateway to the Enterprise IT data center. This 2nd blog addresses the question, “Which SSD is ‘right’ for your workload?” and will help you select the data center solid-state drive with the 'right' write characteristics.
In later blogs, we’ll also address other features in Intel Data Center SSDs like power loss protection. For this blog, let’s look directly at the elephant in the room and talk about write endurance.
To get started, let’s define the term and then explore how IO workload affects an SSD’s endurance. Simply put, endurance is the ability to sustain write activity in a storage medium that has known write limitations. You can think of this in many ways, but I personally like the good old cassette tape analogy. There is a limit to how many times you can play a tape before Peter Frampton just doesn’t sound like himself anymore. The mechanical parts wear and stretch the tape in the process of both playback and recording. Akin to this, the flash memory ( NAND ) media that makes up an SSD also wears out on an atomic level as you read and write to it. From the Intel SSD spec sheets, we rate a drive in DWPD or drive-writes per-day for a 5-year time period. For our high endurance Intel SSD DC S3700 Series we get 10 DWPD and for our standard endurance Intel SSD DC S3500 Series we get 0.3 DWPD. But there’s more to endurance than a simple multiplication of DWPD * capacity = daily writes. Hey, what does DPWD have to do with a worn out Frampton tape still in my vintage Honda Civic?
Here’s the scoop. NAND is a tricky medium and requires some serious electrical gymnastics to make it function in an Enterprise IT role. When the drive is reading, writing, leveling wear, clearing space, and doing other housekeeping operations it produces something we call Write Amplification (Write Amp). At the most basic level, Write Amp is the ratio of host writes to NAND writes. In other words the ratio of writes a computer system made to the device to the number of internal operations the drive needed to accomplish to satisfy those host writes. When we measure endurance in the Intel Data Center family of products, we do so at a Write Amp of about 3.0. This Write Amp corresponds roughly to a 4k block size 100% random write workload across the entire size/span of the drive.
The reader is thinking at this point…. AAACH TOO DEEP! But hang in there, your workload matters because as a general rule anything that decreases Write Amp increases the endurance of the SSD. There are three major things that can decrease Write Amp these include; an increase in block size from 4k, a decrease in randomness from 100%, and use of less than 100% of the capacity of the drive. Conversely, shrinking the block size (since we can’t get any more random than 100%) will increase Write Amp because more housekeeping is required for each operation. Once you get this, it’s more fun than Schrodinger’s cat!
So there it is, your workload has a direct effect on the endurance of your Intel SSD. Intel’s SSDs are tested for that worst case of 4k-100% span-100% random workload, and your workload may be less intense which will allow the drive to function longer! It’s as if playing that Frampton cassette in its entirety (sequentially) or just playing only the 1st song makes the whole tape last longer. With this info under your belt you can measure any particular workload with perfmon, iostat, or your favorite performance tool, then look at those stats in comparison to the benchmark to get a good idea of which drive to purchase and at what capacity. The Intel DC S3700 SSD Series is better for high endurance needs, while the Intel DC S3500 SSD Series is better for those more standard endurance workloads.
This leads us into the additional features I breezed by in the beginning of this blog. We’ll look at some of these in the 3rd blog in this series a few weeks from now, see you then…
Christian Black is a Datacenter Solutions Architect covering the HPC and Big Data space within Intel’s Non-Volatile Memory Solutions Group. He comes from a 23 year career in Enterprise IT and you can follow his travels on Twitter at @RekhunSSDs.