WD SN850
SSD Specification and Info
WD SN850 is an High-End NVMe SSD produced and sold by WD. The device comes with x4 PCIe 4.0/NVMe interface and M.2 form factor - a good fit for both desktop and laptop computers. This SSD has a maximum sequential read-write speed of up to 7000/5300 MB per second, making it ideal for gaming and workstation PCs.
WD SN850 is equiped with DRAM memory, and a TLC NAND flash memory with 112 cell layers.
NAND type
WD SN850 is using a TLC (3 bits per cell) NAND manufacurted by SanDisk with 112 cell layers on top of each other.
The TLC is the most common type of SSD NAND flash memory found on the market at the moment. It is faster, less durable, but still cheaper than the other, more expensive variants - SLC and MLC.
The main advantage of this type of NAND chips is the fact that the cost per gigabyte is much lower, allowing high capacity SSDs at affordable price.
The Controller
WD SN850 is using WD Proprietary SSD controller to connect the NAND memory to the x4 PCIe 4.0/NVMe interface. The controller has Tri-core, 4-ch, 8-CE/ch configuration.
Typically, SSD controllers are microprocessors. In this case we have Tri-core, 4-ch, 8-CE/ch processor responsible for controlling the SSD in such way, so that the data coming from the interface can be stored on to the NAND flash memory.
Some SSDs have simpler controllers with fewer communication channels and less cores.
Among other things, the controller also manages the SLC caching, optimizing the DRAM cache, encryption, LDPC, garbage collection, wear-leveling as well as TRIM
DRAM Cache
WD SN850 has a separate DRAM chip to store the SD mapping tables. DRAM cache speeds up the data access significantly compared to the DRAM-less models.
As soon as the OS requests some data from the SSD, the SSD needs to know exactly where it is on the drive. Because garbage collection moves the data constantly, the controller relies on the mapping tables to locate it.
These tables are stored in DRAM cache, where they are accessed much more quickly than in NAND flash.
Therefore, SSDs with DRAM-less architecture have more random write and read operations. This makes the device perform worse and last shorter if they are not HMB enabled.
HMB Support
There is no HMB architecture available on the WD SN850 to store the mapping tables. The device either doesn't support the architecture or uses DRAM cache.
The HBM is used to reduce the cost of production of NVMe SSDs with DRAM Cache, SSDs with this type of controllers can leverages the host system's DRAM instead of an onboard DRAM chip to host the FTL mapping table used by flash storage.