(S)FTP Synchronization with Compare File content

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Joined: 10 Oct 2019

Post by mayhem • 10 Oct 2019, 01:16


So far I've been using Unison for 2-way sync, but its UI and SSH over unstable network are not that great. Also, symbolic links are not supported on Windows. I'm considering switching to FreeFileSync, but was unable to find the information about internals of syncing large directories over Internet (e.g. 2-way sync of 5TB of data over SFTP, comparing file content). I'm interested in comparing file content rather than just file dates and sizes as data consists of both big multimedia files and source code. What Unison does in this scenario is to start two instances of app, one on local and one on remote computer, and each calculates the hashes of its files, then they exchange data, compute delta and synchronize. So, local computer does not need to initially read the data from remote machine over slow network in order to synchronize by content. I'm wondering how FreeFileSync handles this use case, is it supported at all? I haven't seen the option to start another "hashing" instance on the remote host, only (S)FTP seem to be supported.


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Post by Plerry • 10 Oct 2019, 07:13

FFS does not use remote instances.

It is the strength (and partially the weakness) of FFS that a sync only runs / needs to run on a single computer that just needs to have access to the (possibly remote) left and right file location(s). No software needs to be installed or run on the remote file server.
As installing and setting up software on a (remote) file server is often troublesome, not possible or not permitted, this is a big plus for FFS.

Conversely, a comparison on file content requires all file content to be pulled (downloaded) to the computer running the FFS sync.
Obviously, that creates much more network traffic than when each network location determines the checksums of its local files and only the checksums need to be exchanged for comparison.
When just comparing based on file date and size this is considerably less of an objection.
However, (S)FTP does not always support the preservation of file dates when copying, so comparing by date/size may not always be an option when using (S)FTP.

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Post by mayhem • 11 Oct 2019, 21:31

Yes, just as I suspected. It's a shame, though, and FreeFileSync seems to have a nice UI. I guess I'll have to look into some commercial alternatives like GoodSync...