File system. Drives cache recently read blocks. This isn’t something that’s neesesarily e en visible to the OS.
I decided to benchmark these scenarios out of curiosity, because I hadn't heard of eatmydata before or of force-unsafe-io.
- Docker image: `debian:stable-slim`
- `docker system prune -a` ran before each test.
- Packages installed: `build-essential ca-certificates libssl-dev software-properties-common wget curl git`
Results:
Default Debian settings: 48.32 seconds
force-unsafe-io enabled: 55.86 seconds
eatMyData enabled: 35.04 seconds
I ran it three more times. Plain install was 50s 48s 47s; eatmydata was 46s 46s 46s.
Installing build-essential probably does enough non-dpkg IO to account for the difference (install scripts etc, for example when libc changes and the system scans for running daemons) but it isn’t much and the delta may well be statistically insignificant.