Query re: rolling checksum algorithm of rsync

[Wayne Davison]

On Thu, Feb 10, 2005 at 11:36:51AM +0000, Alun wrote:
> If you're synchronising log files, for example, then you may be able
> to guarantee that all changes to the file happen at the end of it.
> Unfortunately, rsync doesn't give you the opportunity to use this
> extra information to save I/O and bandwidth.

Ftp clients have a resume mode that assumes that the existing data on
the receiver is identical to the start of the data on the sender, and
just starts appending new data onto the end.  It would be possible to
add an option such as "--append" that would do an update like that with
rsync (and would also verify the file's checksum to ensure that it was
really updated properly).  Since it was pretty simple to work that up, I
created a patch for this and checked it into the "patches" dir:

    http://rsync.samba.org/ftp/unpacked/rsync/patches/append.diff

You'll also find it in the latest "nightly" tar file.  It's been only
very lightly tested, so be careful if you fiddle with it.

I'm not sure it really buys us much over a normal --inplace.  It does
lighten the CPU load a bit, but if you're wrong about the data really
just being appends, it forces an extra --inplace update of the file(s)
to correct the problem.

..wayne..