clonetool

Block-level sync for a large file or block device between two machines (or two paths on the same machine), driven from a third, passive "manager" machine. Single static Go binary, no runtime dependencies beyond the system ssh client for remote endpoints. Linux only.

How it works

• You run clonetool sync on the manager. Source, destination, and manager may be three entirely different machines — the manager never reads or writes a single block itself. It only spawns and talks to two control agents (clonetool agent --role control, one per side, local subprocess or over ssh) that do the stat/prepare/transfer work.
• The two agents compare content by SHA-256 hash per fixed-size block. A block is only re-read once: it's hashed from the buffer it was read into, and that same buffer is what gets sent on if it differs — never read twice.
• Source and destination are scanned in parallel. The destination side streams the hash of each block as it computes it, in block order; the source side consumes that stream and immediately reads, compares and (if it differs) sends that block. So the destination scan, the source scan and the transfer of changed blocks all overlap — there is no "scan the whole destination, then start" phase. The status line shows the transfer progress with a trailing scan H/M until the destination fingerprint is complete.
• No state is kept between runs. Every sync re-reads and re-hashes the destination's current content and compares the source against that, so a re-run only moves the blocks that actually differ and nothing needs to be trusted from a previous run. (The destination side always does the destination hashing — the remote sink in push mode, the local destination agent in pull mode.)
• Bulk data goes directly between source and destination, not through the manager. Each run tries:
  1. push — the source agent connects straight to the destination host over SSH and streams changed blocks to it.
  2. pull (fallback) — if push isn't possible (no route/keys in that direction), the destination agent connects to the source host instead and pulls.
  3. If neither direction works, the job fails with both reasons named. Run the manager on the source or destination host, or set up SSH connectivity in at least one direction.
  • When source and destination are both local to the manager, no SSH is used at all — the source agent spawns the write-side helper as a plain local subprocess.
• Self-deploy: if a remote endpoint has no runnable clonetool on PATH (or wherever --remote-bin points), or the one that's there is a different build than this binary (see below), the manager streams this binary to ~/.clonetool/bin/clonetool on that host over the existing SSH connection and uses it — no install, no root. Disable with --deploy=false (a stale or missing binary is then an error, except a merely-stale --remote-bin is used anyway with a warning, since there's no way to correct it). If the copied binary won't execute there (wrong CPU architecture) the error says so; build one for the remote's arch (CGO_ENABLED=0 GOOS=linux GOARCH=arm64 go build) and put it on PATH or pass --remote-bin.
• Version/staleness check: every binary has a build timestamp baked in (see Build below) and clonetool version prints it. Before using a remote clonetool, the manager runs its version there and compares that timestamp against its own — a mismatch (e.g. the remote was self-deployed from an older build of this tool) is treated the same as "not runnable" and triggers the self-deploy above, so the remote binary is kept in sync with whatever you're running locally. Only the build timestamp is compared, not GOARCH — that is expected to differ across a cross-compiled deploy.
• sudo: if reading or writing an endpoint that is a block device fails with a permission error, --sudo=auto (the default) transparently restarts that side's agent — and the helper it spawns on the peer — under sudo. Locally that may prompt for a password on your terminal; on a remote host it uses sudo -n, so passwordless sudo (NOPASSWD) must be configured there (a password prompt can't work — the transfer protocol owns the SSH stdout stream). --sudo=always elevates from the start; --sudo=never never does.
• Sizing rules:
  • Destination is a block device: it can't be resized, so if the source is larger the job fails; otherwise exactly the source's size is synced and the remainder of the device is left untouched.
  • Destination is a regular file: it's truncated (created if missing) to exactly the source's size, growing or shrinking it. Shrinking an existing non-empty file prompts for confirmation unless --yes is passed.

Build

CGO_ENABLED=0 go build -o clonetool .

Cross-compile for another architecture by setting GOARCH (no cgo, so this works from any host):

CGO_ENABLED=0 GOOS=linux GOARCH=arm64 go build -o clonetool .

./build.sh writes linux/amd64 and linux/arm64 binaries into dist/. It also stamps every binary it builds with the same build timestamp (via -ldflags -X main.buildTime=...), which is how the manager tells a stale self-deployed remote binary apart from a current one (see "Version/staleness check" above) — build all your binaries for a given release with one ./build.sh run so they compare equal to each other. A plain go build with no -ldflags leaves the timestamp at "dev", which still works for this check (it only ever compares equal to another literal copy of the same dev binary, never to a real timestamp).

Copy the resulting binary to the manager host. Source and destination hosts get it automatically (see self-deploy above), or place it yourself and point --remote-bin at it. The binary is architecture-specific — cross-compile if your hosts differ.

Usage

clonetool sync --source LOC --dest LOC [options]

LOC is either a local path (/dev/sdb, ./image.bin) or [user@]host:path for a path reached over SSH.

# Same machine
clonetool sync --source /dev/sda --dest /dev/sdb

# Whole disk to an image file
clonetool sync --source /dev/sda --dest /srv/sda.img

# Two remote machines, orchestrated from a third
clonetool sync --source db1:/dev/vdb --dest backup-host:/srv/db1.img

# Re-run any time; only changed blocks move
clonetool sync --source db1:/dev/vdb --dest backup-host:/srv/db1.img

Options:

Flag	Default	Meaning
--block-size	4M	Block size (accepts K/M/G suffixes).
--job	—	Optional label shown in progress/log output.
--yes	off	Don't prompt before shrinking an existing destination file.
--sudo	auto	Block-device privilege escalation: auto (on a permission error), always, or never. Remote elevation needs passwordless sudo.
--deploy	true	Copy this binary to remote hosts that lack a runnable clonetool. --deploy=false to disable.
--connect-timeout	8	SSH connect timeout (seconds) used for the push/pull direction probe.
--ssh	ssh	ssh binary to use.
--ssh-opt	—	Extra -o OPT passed to ssh (repeatable).
--remote-bin	clonetool	Path to clonetool on remote hosts.
--manager-host	local hostname	Address a peer should use to reach this machine, needed only when source or dest is local to the manager and the other side is remote and ends up needing to dial back in (pull fallback).

clonetool version prints the binary's GOOS/GOARCH and build timestamp, e.g. clonetool linux/amd64 build=2024-06-01T12:00:00Z (the timestamp is used internally for the self-deploy staleness check above).

The status line shows read throughput for both sides and write throughput for the destination separately: rd(src) is the source reading and comparing its blocks, rd(dst) is the destination fingerprinting its current content (only while the trailing scan H/M segment is present), and wr(dst) is the destination actually writing changed blocks.

Caveats

• Block-device size detection uses BLKGETSIZE64; the tool is Linux only.
• If a destination path doesn't exist yet, it's created as a regular file — clonetool won't create device nodes, so double-check device paths for typos before running.
• SSH host keys are accepted on first connect (StrictHostKeyChecking=accept-new) and rejected if they later change, same as normal SSH behavior.
• Because every run re-hashes both sides in full, a re-sync costs a full read of source and destination even when little changed — the win is in the bytes transferred, not the bytes read. The two reads run in parallel; the status line shows a trailing scan H/M until the destination fingerprint catches up.
• agent is an internal subcommand spawned automatically by sync; it's not meant to be run by hand, though it will work standalone for debugging.