A few years ago The SSD Guy posted an analogy that Intel’s Jim Pappas uses to illustrate the latency differences between DRAM, an SSD, and an HDD. If we look at DRAM latency to be a single heartbeat, then what happens when we scale that timing up to represent SSDs and HDDs? How many heartbeats would it take to access either one, and what could you do in that time?
I still think it’s a pretty interesting way to make all these latency differences easier to understand.
Just recently I learned of a Rich Report video of a 2015 presentation in which Micron’s Ryan Baxter uses a different and equally interesting analogy based on tomatoes.
Tomatoes aren’t the first thing that comes to my mind when I think about SSDs, but this video may change my way of thinking!
The tomato slide, 9:30 into the presentation, is shown below. It expresses the time differences between a DRAM, SSD, and HDD access to the time it takes to get a tomato for a pizza recipe.
- A DRAM access is like opening your refrigerator and grabbing a tomato, which he estimates will take about 6 seconds
- An SSD access is like driving 2.2 hours to a country produce stand to get that tomato
- An HDD access would be like growing a tomato plant from a seed and waiting for a tomato to develop and ripen 46 days later
Jim Pappas’ analogy uses the very simple rule of thumb that an SSD access takes 1,000 times as long as a DRAM access, and that an HDD access takes 1,000,000 times as long as DRAM. Ryan Baxter’s analogy appears to use hard numbers because the ratios to get to his numbers are 1,320 and 662,400. All in all, though, they’re pretty close, and either would work for a performance estimate.