aria2 speed & BitTorrent optimization

Before you start

• aria2 installed and on your PATH.
• Know your storage type (SSD or HDD) and RAM footprint — they decide the right values.
• Optional: for BitTorrent, a router where you can forward one TCP+UDP port.

Change these in ~/.aria2/aria2.conf, %USERPROFILE%\.aria2\aria2.conf, or pass them as CLI flags. See the options reference.

Where speed really comes from

“aria2 is slow” almost always means one of four bottlenecks. Fix them in order:

1. Connection count. aria2's HTTP default is only max-connection-per-server=1. Raising it is the single biggest win on throttled links.
2. Peer/tracker health (BitTorrent). Dead trackers or no forwarded port = no peers = 0 KB/s.
3. Disk I/O. A saturated HDD turns a fast download into a stall. Preallocation and a disk cache fix this.
4. OS limits. File-descriptor and TCP tuning matter once you push thousands of connections.

Also remember the ceiling isn't yours alone: server-side throttling, peering, and your line rate all cap the result. You can only remove your side's bottlenecks.

HTTP / FTP tuning

max-concurrent-downloads=5
max-connection-per-server=16
min-split-size=1M
split=16
max-tries=5
retry-wait=5
timeout=30
connect-timeout=30
lowest-speed-limit=10K
file-allocation=falloc

If a server returns 429/403 or drops connections, lower -x/-s to 4–8 — sometimes “polite” is faster than “aggressive”.

BitTorrent tuning

The commands below are accurate and safe. The most common real cause of 0 KB/s is simply no reachable peers: no forwarded port and/or dead trackers.

# Discovery
enable-dht=true
bt-enable-lpd=true
enable-peer-exchange=true
listen-port=6888
dht-listen-port=6888

# Swarm scaling
bt-max-peers=128
bt-request-peer-speed-limit=50K     # drop slow, stalled peers
bt-tracker-connect-timeout=5
bt-tracker-timeout=5

# Seeding
seed-ratio=1.0                      # 0 = seed forever
seed-time=0
bt-save-metadata=true
follow-torrent=true

Forward one port for TCP and UDP (6888 above) on your router so peers can connect to you — otherwise you only reach out, which limits speed. Keep a fresh tracker list in bt-tracker; grab one from the tracker list.

Encryption (bt-require-crypto=true) can help with ISP shaping but may exclude some peers — test both ways. On private trackers, keep PEX/DHT off if the rules require it.

Disk & filesystem tuning

aria2 preallocates files before writing. The default is prealloc, which zero-fills the file and can pause the whole system on large downloads. Pick based on your storage:

file-allocation=falloc        # ext4/XFS/btrfs/APFS
disk-cache=64M                # batch writes in RAM (default 16M)
no-file-allocation-limit=64M  # preallocate only files above this size (optional)

• Disk cache by RAM: 16–32M (1–2 GB RAM), 64M (4–8 GB — the sweet spot), 128M (16 GB+).
• HDD + many connections is the classic stall: raise disk-cache and use falloc; consider a small SSD cache.
• etx4/XFS handle falloc well; network shares (SMB/NFS) may not — test with a large file.

OS & network tuning

• File descriptors: 16 connections per file adds up fast. Raise the limit: ulimit -n 65536 (Linux) before starting aria2. For a service, set LimitNOFILE in the systemd unit.
• TCP congestion control: BBR often improves throughput on lossy links. Enable with sysctl net.ipv4.tcp_congestion_control=bbr (if available).
• Prefer wired + IPv4/IPv6 sanity: if IPv6 is broken on your host, set disable-ipv6=true to avoid slow fallbacks.
• Proxies: a slow proxy caps everything; measure with and without --all-proxy.

# Linux: raise fd limit for the current shell, then run aria2
ulimit -n 65536
aria2c --max-concurrent-downloads=5 --max-connection-per-server=16 URL

Interactive hardware-optimized config generator

Pick your hardware to generate a tailored performance block for aria2.conf:

The generated block is a starting point — measure (next section) and adjust.

Measure & benchmark

Don't guess — measure. Two easy methods:

# 1. Live summary every 5 seconds from aria2 itself
aria2c --summary-interval=5 -x 16 -s 16 https://example.com/large.iso

# 2. Query a running daemon's current speed over JSON-RPC
curl -s http://127.0.0.1:6800/jsonrpc -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":"1","method":"aria2.getGlobalStat","params":["token:SECRET"]}'

• Use a large file from a host that isn't itself the bottleneck (a big distro mirror or a public test file).
• Change one variable at a time (-x, then -s, then disk-cache) and compare.
• Watch disk usage during the test — if it pegs at 100%, disk I/O is your limit, not the network.

Troubleshooting & FAQ

Related resources

• aria2.conf generator — build a full config visually
• Options reference — every flag used here, with examples
• BitTorrent tracker list — keep swarms alive
• yt-dlp + aria2 — accelerate media downloads
• Troubleshooting — common aria2 errors with exact fixes
• .aria2 file guide — resume and recover partial downloads