This request has been added to the tracker:
Issue 1566 – Connection compression with FTP protocol

Good point. True round trips is a huge cost.

This could be fixed with a recursive ls -alsr on filezilla server and winscp, but that
would be a non-standard command. This way the whole file structure could be compressed
and sent in one go.

However, most files we noticed that were being sync'd were html, sql, csv and mssql bak
files.

Uncompressed it was trying to transfer 4GB every 6 hours. The CSV, SQL and MSSQL bakups
are normally over 1GB. Compressed it was less than 500k. That's over 16GB vs 2GB over a
day from one country to another.

This does result in a huge speed up.

Ofcourse scripts could be written to post and pre compress, but this becomes complex, with
timing and all that unless we used a sshd server, and results in many processes and many
possible points of failures.

What makes you believe that the compression would speed up the synchronization? Not that it cannot, but in most cases, the synchronization is slow because it involves a huge amount of round-trips to/from the server. Compression would not help with that.

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Contains a SSHZLIB.C to do mode z.

WinSCP already seems to compile with libz.

So just a case of decoding file on download... ?

I noticed the Filezilla server supported it, so assumed the client supported it.
I did look at the Filezilla client code after posting, and you are right. I could not find the [m]MODE Z[/m] code.
The great sync feature WinSCP is much slower without compression and uses too much bandwidth.
Do you think it would be difficult to add? I could try assist, or try get some donations going.
It would probably need an exception list so [m]*.jpg[/m], [m]*.png[/m], etc. do not get compressed.
Voyager FTP and core FTP appears to support [m]MODE Z[/m], but sync modules there are not as good as in WinSCP.

That's actually no demand for this feature, that's why WinSCP does not support it.

Also, are you sure that FileZilla supports MODE Z?
#2977 (MODE Z Compression option in filezilla 3) – FileZilla

Why does WinSCP not support Mode Z compress when Filezilla does, and both are supposed to be based on the same code?

Are there known major hurdles to supporting it, or is it just a weekend of coding?

Lack of compression makes the great sync feature operate slowly..