Pacman ParallelDownloads and systemd-journald interaction under concurrent package installations

Goal

Reduce the observable latency that occurs when multiple pacman package installation requests run concurrently, while preserving the integrity of the package database.

Constraints and uncertainty

Pacman serializes write access via a database lock file; the ParallelDownloads option only parallelizes the download phase, so concurrent processes still block on the lock. Under high concurrency, systemd‑journald disk I/O can become a bottleneck, adding further delay. The optimal value for ParallelDownloads and the interaction with journald’s SystemMaxUse setting are not fully characterized, and it remains unresolved whether pacman should introduce a transaction queue or allow safe concurrent database writes.

What is the measurable impact of lock contention on installation latency when ParallelDownloads is enabled? Should pacman provide a queuing mechanism to serialize transactions without blocking downstream services? How does adjusting systemd‑journald disk usage limits influence the observed latency under concurrent pacman runs?