These docs describe unreleased changes on master. Read the latest stable documentation.

Server setup

Start by checking TCP access and the source and destination filesystems. Use test data when comparing settings.

Make TCP reachable

Syq listens on one available port in 47600–47699 during a copy. Choose another range with --tcp-ports LO-HI. For a server using ufw, an administrator can allow a trusted client:

sudo ufw allow from <trusted-client-address> to any port 47600:47699 proto tcp

Allow the range in any cloud firewall too. Check the transport with syq cp -vv --stats. Ordinary copies fall back to SSH on the same route when TCP is blocked. Copies authorized through another machine require direct encrypted TCP.

Tailscale

Tailscale can make servers reachable across NAT and firewalls. Allow syq’s data ports through the host firewall and tailnet rules; syq can discover Tailscale addresses. Use tailscale status to check whether the connection is direct or relayed. See Tailscale’s performance guide.

Test congestion control

On Linux, check which TCP congestion-control algorithms are available and allowed on both endpoints:

sysctl net.ipv4.tcp_available_congestion_control
sysctl net.ipv4.tcp_allowed_congestion_control

If bbr is listed in both, compare it with cubic on your route:

syq cp --tcp-congestion bbr --stats data --to server --into /backup

Use the same data, a fresh destination, and both transfer directions. The option applies to syq’s TCP sockets. If BBR is missing, see the administrator setup guidance.

Let SSH connections start promptly

OpenSSH’s MaxStartups limit can slow parallel logins. For servers handling parallel transfers, an administrator can consider:

MaxStartups 100:30:200

This allows 100 unauthenticated connections before random rejection begins, and rejects all new ones at 200. It also admits larger bursts from unrelated clients. MaxSessions controls channels sharing a connection; very low values can force extra logins.

Validate changes with sshd -t, then reload SSH using your system’s procedure. Keep an administrative session open. See OpenSSH’s settings.

Check local storage placement

On Linux, inspect the source and destination filesystems:

findmnt -T /path/to/source -o TARGET,SOURCE,FSTYPE,OPTIONS
findmnt -T /path/to/destination-parent -o TARGET,SOURCE,FSTYPE,OPTIONS

Use an existing destination parent and run remote-path checks on the machine that owns the path. Copies within a filesystem supporting cloning can share storage while remaining independently writable. See local copies and NFS for filesystem and mount considerations.

Measure and track improvements

Use syq-bench for repeatable comparisons. Record the commands, versions, mounts, cache state, and other load. Keep reporting and flush settings consistent across runs. See performance tuning to compare worker counts and request sizes.