Many solid-state drives write into a fast cache first. When it fills, the write rate falls to a much lower level. A short test never reaches that point.
- SystemCheck's sustained write mode writes one stream of data for up to 120 seconds or 8 GiB, whichever comes first, and plots the rate against bytes written.
- It calls a cliff a fall below 50% of the rate of the first GiB, held for at least 3 chunks in a row. That line is ours, not a standard.
- It measures the browser's storage path, not the drive directly. The figure is what a web app can get here, not the drive's rated speed.
The short answer
A drive's advertised write speed is usually its cache speed. To see the sustained speed you have to write more than the cache holds. SystemCheck's storage test has a sustained write mode that keeps writing and shows where, if anywhere, the rate falls.
Why SSDs slow down mid-write
Many consumer SSDs store several bits per cell (TLC or QLC), which is slower to write. To hide that, they treat part of the drive as a faster single-bit cache and write there first, moving the data to its final place later. Once the cache is full, new writes go at the slower native speed. How big the cache is varies with the drive and often with how full it is.
How the browser test works
The test writes a file into the browser's private storage for this site, in large chunks, from a worker, timing each write call. The page asks the browser to make the storage persistent before it starts, and the amount it writes is capped by the space the browser says is free.
- Cap: 8 GiB or 80% of the space the browser reports as available, whichever is less.
- Verify: every 8th chunk is read back and compared with what was written.
- Clean up: the file is deleted at the end.
- A hidden tab pauses the write, and the paused time is flagged.
What it cannot see
The browser and the operating system may buffer writes, and the browser may grant less space than it estimated, in which case the write ends there and what was measured stands. If the cap is smaller than the drive's cache, no cliff is reached and the test says so; that is not proof there is none. Drive health, its model and its temperature are not visible to a web page; the drive maker's tool or CrystalDiskInfo on Windows reads them.
Terms used here
Questions
- Will the sustained write test wear out my SSD?
- It writes up to 8 GiB once and deletes it. That is a small amount next to the total writes a drive is built for, but it is not nothing, so there is no reason to run it repeatedly.
- Why is the browser's figure lower than my drive's rating?
- The rating is usually measured with a dedicated tool writing straight to the drive, often into its cache. The browser adds its own storage layer, so its figure is what a web app can get, which is a different thing.