where is backup-dir rooted?

[Alan Chandler]

The current man text for rsync is rather sparse in what it tells you 
about backup-dir

This is from my debian installation - which appears to have come from 
the source here


--backup-dir=DIR

In combination with the --backup option, this tells rsync to
store all backups in the specified directory on the receiving
side. This can be used for incremental backups. You can addi‐
tionally specify a backup suffix using the --suffix option (oth‐
erwise the files backed up in the specified directory will keep
their original filenames).


the "Specified Directory" and "Receiving Side" is the best it gets at 
telling me.

if I have

rsync -a --delete --backup --backup-dir=/some/backup/path 
/path/to/my/files/ another-machine::module/my/backup/

where is /some/backup/path rooted? if is was instead some/backup/parth 
(without the leading slash) where is it rooted, does a trailing slash 
have any relevance?

I am assuming that "something" is related to another--machine, but can 
it be a module specification followed by a directory, or must it just be 
a directory under the same module as the destination directory?

I have googled for the term backup-dir, but can find nothing further or 
more specific.

[What I am trying to achieve is to backup (part of) my home directory to 
a backup server such that

a) I have a reasonably up to date (within a day if I do it overnight) of 
my current state
b) If I have deleted or updated a file the old version of it gets placed 
into a special "snapshot directory" away from this backup.

A set of overnight cron jobs turn my snapshot directory into (over time) 
a daily, weekly and then monthly archive of what gets placed there. So 
if I change a file every couple of days, I have snapshots of previous 
states stretching back from when I first created it]

-- 
Alan Chandler
http://www.chandlerfamily.org.uk