Is zsync really useful for -tcz?

[bmarkus]

In the repo there is a .zsync file for .tcz's. At the zsync site it is written:

If the content of a file is compressed, but not in a simple gzip format, bear in mind that zsync may not be effective. Each compressed stream can typically only be efficiently updated via the rsync method if it is either completely unchanged, or the compression has been made rsync-friendly (with, for example, gzip --rsync).

So, for example, zsync is useless for individual Debian or RPM package files, and is useless for bzip2 files. But it might still be useful for an ISO containing lots of such files, if, say, some packages have changed but some are completely unchanged.

Reading zsync docs my understanding is that it is useless for .tcz and works only for gzipped files. Am I wrong?

[curaga]

No, it is useful, even though not perfect due to the compression.

In my tests back then when introducing zsync, a typical admin change of editing the startup script caused a traffic of ~20kb, irrespective of the extension size (biggest tested was a few mb).


If you'd like to test this, just remove the output redirect from the zsync commands in tce-load.

[bmarkus]

Which zsyncmake command line switches used creating .zsync for .tcz?

[curaga]

None - "zsyncmake file.tcz" run in the same directory.

It does post a warning to use the -u option, but since we want a relative path to the same dir, the default behavior is ok.


edit: That should be one of the update scripts and not tce-load in the above post.

[bmarkus]

Thanks, will add .zsync  when updating and check how much is saved.