This improves performance when running with fsync enabled. The key
observation is that the OS may coalesce multiple concurrent fsyncs, so
we gain some performance by issuing them in parallell. Since they run
from the finisher routine, a simple fix is to run multiple finisher
routines. That's the `after` line in the graph below. The other step is
to do the same for the directory fsyncs, issuing them concurrently with
a limiter. In both cases I used the Copiers value as the concurrency
factor. Additionally, add some buffering to the channels between
routines to minimise stalls where a routine needs to wait for another.
This is the `after2` line.
All in all, this speeds up syncing 25k tiny files from 190s to 130s, a
30% improvement.
<img width="821" height="540" alt="Screenshot 2026-07-25 at 22 58 34"
src="https://github.com/user-attachments/assets/18272c89-a99e-4ee4-9b5e-b278967ea7c6"
/>
---------
Signed-off-by: Jakob Borg <jakob@kastelo.net>
Jakob BorgandGitHub
952da4224b