fast-copy vs rsync
rsync is the standard for a reason, and for repeated incremental syncs it still is — delta transfer and --link-dest are unbeatable there. But for a first full copy of a large tree, rsync reads files in directory order and pays a seek penalty on every small file. fast-copy sorts reads by physical disk offset and bundles small files into tar streams, which is where the gap below comes from.
The measured difference
12,347 files, 110.9 MB (9 KB average), rotational XFS source to SSD destination. Page cache dropped before every run; timing includes sync -f, so this is a durable copy, not a buffer race. Best of three interleaved rounds; dedup off so both tools write the same bytes:
| Tool | Time | Throughput |
|---|---|---|
| fast-copy (verify ON) | 8.19 s | 13.5 MB/s · 1,507 files/s |
| rsync -a | 14.14 s | 7.8 MB/s · 873 files/s |
With dedup on, the same job finishes in 5.93 s — but that is no longer an apples-to-apples write comparison. Full methodology and every command here.
Feature differences that matter
| fast-copy | rsync | |
|---|---|---|
| Read order on disk | physical-offset sorted (FIEMAP) | directory order |
| Content dedup → hard links | yes (xxHash-128, cached across runs) | no (checksum is for delta, not dedup) |
| Needs software on the remote | only ssh + tar (BusyBox/Synology OK) | rsync binary on both ends |
| Post-copy verification | on by default | --checksum re-read (opt-in, slow) |
| Pre-flight space check | yes, allocated-size aware | no |
| Cloud endpoints (S3 / Azure / GCS) | built in | no |
| Windows | native | cygwin/WSL only |
When rsync is still the better tool
Honest answer: repeated syncs of mostly-unchanged trees. rsync's delta algorithm transfers only changed blocks, and --link-dest snapshot chains are a proven backup pattern. fast-copy skips unchanged files entirely (hash cache), but it does not do sub-file deltas. On NVMe-to-NVMe the block-ordering advantage also mostly disappears — there, the tools are close for full copies and rsync wins the incremental case.
Also compared: fast-copy vs cp · fast-copy vs scp / SFTP · fast-copy vs Robocopy · fast-copy vs FastCopy · fast-copy vs TeraCopy · full benchmark methodology