SLC cache
An SLC cache is part of a TLC or QLC drive's flash run at one bit per cell, so writes land fast until it fills and speed drops to the native rate.
Also known as SLC write cache, write cache, pseudo-SLC, cache cliff
Checked by a SystemCheck tool
The USB drive test checks this in your browser, on your own device. The paragraphs below say what it reads and what a browser cannot see.
Flash stores bits as charge levels in cells. Single-level cell (SLC) flash stores one bit per cell and writes fastest; TLC stores three and QLC four, packing more capacity into the same silicon but writing more slowly. Most consumer solid-state drives use TLC or QLC and run part of it in single-bit mode as a cache. Writes land there first at high speed, and the drive moves the data to its denser native form later, when it is idle.
A rated sequential write speed is usually the cache's speed. Copy more than the cache holds in one go and the speed falls, often sharply, to what the flash can sustain directly. On many drives the size of the cache depends on how full the drive is, and spec sheets rarely print it, which is why the same drive can write a small file quickly and a large one slowly.
A browser cannot read the drive model or its cache, but it can show the cliff. The storage test's sustained write mode times one large file chunk by chunk and draws the rate over bytes written, and the USB drive test flags a fall when later files write at under 50% of the early median and stay there, our line. USB sticks and SD cards show the same shape from their own write caches.
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This page defines the term. The pages below explain it, diagnose it and measure it.
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