SSDs often, but not always, come with DRAM for caching. While it’s possible to embed the DRAM into the controller, more commonly this memory lives in one or more packages external to the controller. These can be standard DDR3 or DDR4, or be a low-power form of DRAM to reduce power consumption. They are not dissimilar to what you find on your system’s memory sticks. However, for usage on a SSD the advantage of latency is more important than bandwidth.
While DRAM can be used as a write cache, most typically it acts as a metadata cache for the SSD. Metadata, or data about data, includes mapping and addressing information to help the SSD translate between physical and logical data locations. This is known as a Lookup Table (LUT). Metadata can also include information used for wear-leveling, among other things. Usually the most recently used and most often used metadata will appear in DRAM, and the SSD always has a complete copy in the NAND flash for committed data.
The Host Memory Buffer, or HMB (not to be confused with HBM), feature of the NVMe specification allows a DRAM-less drive to utilize some system memory for this caching. This may only be tens of megabytes which is adequate for normal usage. DRAM-less drives may have other drawbacks like higher write amplification, which reduces endurance, or weaker sustained performance. This is especially true for SATA and QLC drives. However, these drives are improving with intelligent controllers.
Most of our drives come with DRAM for high performance. The amount varies from drive to drive and based on capacity, but is always plentiful for even heavy workloads. Our Rocket 2242 lacks DRAM but supports HMB, in order to help keep the form factor small. It’s not unusual for smaller NVMe drives to be DRAM-less due to space limitations. Our goal is always to have the best performance possible, balanced with other factors to generate our diverse product stack.
Relative access latencies - Source: Intel Patent US 10929285
Latency sources including LUT - Source: Marvell, FMS 2015
For more information on HMB and SSD DRAM, please see Phison's blog on the subject.
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