ParallelDownloads in pacman 6.0: Picking a Per-Mirror Concurrency Limit

Answer

The ParallelDownloads setting in pacman 6.0 is a global cap on the number of simultaneous download streams within a single transaction, not a per‑mirror limit.

Confirmed facts

• Pacman schedules up to N payloads, each assigned to the first usable mirror in the list and falls back on failure.
• The option is commented out by default; the packaged example value is 5.
• Signature and database checks happen after download, so concurrency cannot break transaction integrity.
• Throttling appears as stalled or sharply decelerating transfers on a single host rather than clean HTTP errors.

Likely explanation / practical guidance

1. Check your installed pacman version and whether the option is active:

pacman --version
grep -n ParallelDownloads /etc/pacman.conf

2. If the line is missing or commented, enable it and set a starting value of 5 (a safe middle ground):

   sudo sed -i 's/^#ParallelDownloads.*/ParallelDownloads = 5/' /etc/pacman.conf
   # or add the line if absent
   

3. Run a typical update and watch the output for parallel progress bars and per‑file speeds:

   sudo pacman -Syu --verbose
   

4. If you see smooth, steady speeds close to your link capacity, the value is adequate. If transfers stall or drop sharply while the overall speed stays low, lower the value (e.g., to 3) and repeat; the disappearance of stalls indicates mirror‑side throttling.
5. Re‑evaluate the setting whenever:
• You regenerate or change your mirror list (after reflector or rankmirrors).
• Pacman is upgraded – check for /etc/pacman.conf.pacnew and merge any new default.
• You observe persistent stalls that vanish when the concurrency is reduced.

Assumptions and uncertainty

The behavior described applies to pacman 6.0 and later releases that retain the same implementation. Exact parsing of invalid values or the default when the option is absent may differ; consult man pacman.conf for your specific version.