https://documents.westerndigital.com/content/dam/doc-library/en_us/assets/public/western-digital/product/internal-drives/wd-blue-ssd/user-manual-wd-blue-3d-nand-sata-ssd.pdf
Tiered Caching
The WD Blue 3D NAND SATA SSD uses a tiered caching structure to improve write
performance and endurance.
Modern operating systems typically access the storage device using small access
blocks; the majority of these being 4KB. These access blocks are incongruent with
the physical block size (less than 1MB) of the newer generation of
flash
memory
technology, and writing directly to the TLC array is also slower than writing to SLC
blocks. To overcome these hurdles, the WD Blue 3D NAND SATA SSD employs three
storage layers:
▪ Volatile cache — DDR DRAM cache
▪ Tiered Caching structure — A non-volatile
flash
write cache
▪ Mass storage — TLC NAND
flash
The tiered caching structure is a pool of X1 blocks which are used as write cache to
accumulate and consolidate all writes at high speed. It works in the background,
then
flushes
them into the larger X3 storage blocks and uses optimized write
transaction sizes to maximize endurance.
All host data is written to X1 blocks to achieve the highest performance. Three X1
blocks are copied sequentially into one X3 block during the
flush
to mass storage
when exceeding their threshold.