Diagnosing Pacman ParallelDownloads Failures on Arch Linux

Repeated failed retrieving file errors during pacman -Syu

Updates start, download speeds spike then collapse, and pacman ends with warning: failed to retrieve some files and error: failed retrieving file .... The useful takeaway is that ParallelDownloads in /etc/pacman.conf can open more simultaneous connections than a mirror allows, causing resets and throttling. Tuning the parallel count or mirror selection usually restores stable downloads.

Recognizable condition

Run an update from a terminal as your regular user with sudo for pacman:

sudo pacman -Syu

Signs of a ParallelDownloads problem:

• Intermittent error: failed retrieving file ... from ... for multiple packages in one run.
• Overall throughput drops after the first few files, or connections reset.
• The same run succeeds after setting ParallelDownloads = 0 or lowering the value.

Pacman logs the pattern in /var/log/pacman.log. Look for repeated failed retrieving entries clustered around the same update.

sudo grep -i 'failed retrieving' /var/log/pacman.log

This command requires sudo to read the log and shows whether failures are recent and recurring.

Cause / diagnostic table

Observation	Likely cause	Why it matters
Failures disappear with ParallelDownloads = 0	Too many simultaneous connections for the mirror	Mirrors enforce per-IP connection limits; excess opens cause resets or temporary throttling
Failures on one mirror, fine on another	Mirror-specific rate or connection limits	Some mirrors are slower or block high parallelism
Failures only with ParallelDownloads > 10	Aggressive parallelism	High values increase contention and risk bans
Slow but no errors with low ParallelDownloads	Parallelism too low or mirror slow	Benefit of parallelism is lost, update time grows

Ordered checks

1. Confirm current setting

Check the active value in /etc/pacman.conf. Edit as root.

grep -E '^#?ParallelDownloads' /etc/pacman.conf

If the line is commented out, pacman uses the default of 1.

2. Inspect recent pacman log

Identify if failures correlate with ParallelDownloads changes.

sudo tail -n 200 /var/log/pacman.log | grep -i 'failed retrieving'

Risk: log file is large; tail limits output.

3. Check mirror responsiveness

Use reflector to assess current mirrors. Run as user; reflector writes to /etc/pacman.d/mirrorlist when used with sudo.

reflector --verbose --latest 5 --sort rate

This shows which mirrors respond quickly. A mirror that is fast in rate sort is more likely to tolerate moderate parallelism.

4. Test with conservative parallelism

Temporarily set a conservative value to test behavior without committing to a permanent mirror change.

sudo nano /etc/pacman.conf

Find the [options] section and set:

ParallelDownloads = 3

Save and run a small update to observe stability.

Fixes tied to findings

Finding: failures stop when ParallelDownloads is lowered

Keep a conservative value. Many users find 3-5 stable across a range of mirrors.

ParallelDownloads = 3

Rollback: restore the previous number or comment the line to revert to default 1.

Finding: failures persist on a specific mirror

Refresh mirror list toward faster, well-provisioned mirrors.

sudo reflector --latest 20 --sort rate --save /etc/pacman.d/mirrorlist

Risk: changing mirrors changes package sources; verify the list contains official Arch mirrors.

Finding: disabling parallelism fixes it completely

Set ParallelDownloads = 0 to disable parallelism. This eliminates connection pressure at the cost of longer updates.

Limitation: setting too low negates the benefit of parallelism and increases overall update time. Setting too high can cause temporary bans or throttling from mirrors.

Verification

• Run sudo pacman -Syu and observe stable download speeds with no failed retrieving file messages.
• Check /var/log/pacman.log for absence of new failed retrieving entries related to the updated packages.
• Confirm mirror health with reflector --verbose --latest 5 --sort rate to ensure selected mirrors can handle the chosen parallel count.

Escalation criteria

Escalate beyond local tuning if:

• Failures occur with ParallelDownloads = 0 and multiple refreshed mirrors.
• Errors reference signature or database issues rather than retrieval.
• Network tools show packet loss or DNS failures independent of pacman.

In those cases, check system time, network connectivity, and Arch mirror status before changing pacman configuration further.