SSD SLC Cache Explained: Why Write Speed Changes During Large Transfers
Short benchmark bursts can fit inside a fast cache while long writes expose slower sustained NAND speed.
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In this articleWhat the cache doesWhy the fast phase endsDynamic cache size changesBenchmark implication
What the cache does
Many TLC and QLC SSDs temporarily operate part of their NAND in a faster single-bit style. This absorbs bursts and improves perceived write speed.
Why the fast phase ends
Once the available cache is filled, new data must be committed at the drive’s underlying sustained rate. That rate can be much lower than the initial burst.
Dynamic cache size changes
Some drives use more cache when more free space is available. A nearly full drive may therefore leave less room for high-speed burst writes.
Benchmark implication
A small test file may mostly measure cache behavior. A large sustained transfer answers a different question and can produce a very different number without either result being “wrong.”
Practical takeaway: interpret storage data in context, protect important files with independent backups, and compare trends rather than relying on a single screenshot.