Last updated: 2026-07-09
rsync has mirrored folders since 1996 for a reason: bidirectional sync, a rolling-checksum delta algorithm that transfers only the bytes that actually changed inside a file, and decades of flags for every edge case. If you already have SSH access and know rsync, it is very hard to beat.
Relayium's sync command covers a narrower job — a one-way incremental mirror — but it removes a step rsync assumes you've already done: setting up SSH. It runs over SSH if you already have it, or connects two machines directly with no SSH server at all. This article compares the two honestly; sync is not a full rsync replacement, and the FAQ says so plainly.
rsync is bidirectional — either side can be the source — and its delta-transfer algorithm compares blocks of a file with a rolling checksum, so a small edit to a huge file only sends the changed blocks, not the whole file again. That, plus decades of flags (--exclude, --link-dest for hard-link snapshots, compression, bandwidth limits, and more), make it the right tool for a lot of jobs.
It's also everywhere: most Linux and macOS systems ship it, and it works over an SSH connection you've probably already configured. None of that is something Relayium sync tries to replace.
relayium sync <src...> <dest> [--delete] [--watch] is a one-way incremental mirror: it compares each file's size and modification time, skips what already matches, and sends the rest.
It works over two transports. Point it at user@host:/dest and it runs over SSH — but relayium must already be installed on the remote; unlike push, there's no tar fallback for sync. Point it at relayium://host[:port] instead and it skips SSH entirely: a pinned TLS 1.3 connection straight to a relayium serve listener on the other machine, authenticated by that machine's fingerprint (approved once, remembered after). That second path is the real convenience: no sshd to configure, no SSH keys to manage — just two machines with relayium installed and a port open between them.
relayium sync ./photos user@host:/backup/photos # over SSHrelayium sync ./photos relayium://203.0.113.9:9031 # daemon direct, no SSHBy default sync only adds and updates — it never deletes anything on its own. Pass --delete to mirror deletions too, but the receiver has to opt in: it must be running relayium serve --allow-delete. If it isn't, the delete request is refused and reported back to the sender rather than silently ignored.
For a folder that changes continuously, --watch keeps sync running after the first pass and re-mirrors whenever a file changes under the source — rsync has nothing built in for this; you'd normally reach for a cron job or a wrapper like lsyncd.
relayium serve --dir /backup --port 9031 --allow-deleterelayium sync ./photos relayium://203.0.113.9:9031 --delete --watchEvery file is verified end to end with a SHA-256 hash, and a transfer that gets cut off mid-file resumes from the byte offset already on disk instead of restarting — useful for the same reasons it is in rsync.
Be clear about what resume is not: it isn't rsync's delta algorithm. If a file is already fully present but has a different size or timestamp, sync retransmits it whole rather than diffing changed blocks inside it. For one huge file that changes slightly and often, rsync's delta transfer will move less data. sync also only handles regular files — symlinks and special files are skipped, so anything relying on those needs handling separately.
Pick rsync when you need sync to go both ways, when you already have SSH set up and don't want another daemon, when you need fine-grained control (excludes, filters, hard-link snapshots, bandwidth limits, compression), or when a file changes slightly and often and delta-transfer saves real bandwidth. Relayium sync doesn't try to cover any of that.
No. It only does one-way mirroring and has no block-level delta algorithm like rsync — a file with a different size or timestamp is retransmitted whole, and it has far fewer flags than rsync. If you need bidirectional sync, fine-grained filter control, or want to lean on rsync's mature ecosystem, rsync is still the better tool.
No. Daemon direct (relayium://host:port) skips SSH entirely: a pinned TLS connection, authenticated by the receiver's fingerprint, approved once and remembered after. You can also go over SSH (user@host:path), but relayium must be installed on the remote for that — and unlike push, there's no tar fallback for sync.
Only if you pass --delete and the receiver's serve is running with --allow-delete. Otherwise the delete request is refused and reported back to the sender, not silently ignored.
Yes, with --watch. After the initial mirror, sync keeps running and re-mirrors (debounced) whenever a file changes under the source.
Yes. The Relayium CLI is completely free, and no mode — sync included — needs an account.
Mirror a folder between two of your own machines — no SSH server required.
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