[Question] Compression for local file transfers

[jb2170]

Hi, sorry I couldn't find this in the documentation so I'll ask:

Does the -z (compression) flag have any effect for local file transfers, that is say does rsync -az foo/ bar/ use compression?

I find myself sometimes doubting whether I should use -vzaH or -vaH as my muscle-memory go-to default flags, whether it's a local-to-local or local-to-remote setup. It seems a bit superfluous if rsync does use compression for local-to-local file transfers right? It'd be nice to know that it doesn't and I can use -z freely without thinking about possible wasted time / energy compressing and decompressing unnecessarily.

Note: I'm not talking about say rsync -az foo/ localhost:~/bar/, for which I would guess compression to be used as per any rsync-over-ssh usage that uses -z, agnostic of whether the host and target machine are the same.

[WayneD]

You're probably wasting cpu on a fast socket or pipe. While it compresses the pipe data on a local transfer, it's not needed. You can see its effect by requesting --stats.

[jb2170]

What's the point of compression for a local file transfer? Is there something I'm not thinking about? Why is rsync's behaviour not to just ignore -z flags for local file transfers?

[WayneD]

Because you asked for it. Why should it not do what you asked?

[jb2170]

It seems pointless and a waste of energy. What's to stop one from changing the behaviour from
--compress, -z compress file data during the transfer
to
--compress, -z compress file data during the transfer if transferring over a network

That would allow one to write rsync -az "${SRC}" "${DEST}" in a script and not care if say SRC is of the form local-path/ or user@host:~remote-path/