How does rsync's --inplace option work with --backup?

It's almost impossible to refer to a file by its inode. To open a file, it is necessary to open a reference to a file name in a directory. Then you have a file handle which is now independent of the file name (which is why a file can be removed from the filesystem but still be open and active). So, let's assume the file is being kept open throughout the life of the application. You now need to replace the content of this file using rsync.

Normally, rsync will create a temporary copy of the target file alongside any existing instance, and then at the last moment it will delete (or backup) the original and switch in the replacement. This behaviour can be modified with the --inplace option, such that instead of creating a new copy rsync will write to the actual target.

Now, you've also specified --backup, so rsync does the right thing and creates a copy for the backup before allowing --inplace to update the original instance. You can see this with a short example:

# Prepare scenario
mkdir /tmp/624404
cd /tmp/624404
date >src
cp -p src dst

# Initial files, with inodes
ls -li src dst
149172 -rw-r--r-- 1 roaima 29 Dec 14 11:49 dst
137559 -rw-r--r-- 1 roaima 29 Dec 14 11:49 src

# Update, copy, and list
date >src
rsync --times --inplace --backup src dst
ls -li
total 12
149172 -rw-r--r-- 1 roaima 29 Dec 14 11:50 dst
149194 -rw-r--r-- 1 roaima 29 Dec 14 11:49 dst~
137559 -rw-r--r-- 1 roaima 29 Dec 14 11:50 src

# Update again, copy, and list
date >src
rsync --times --inplace --backup src dst
ls -li
total 12
149172 -rw-r--r-- 1 roaima 29 Dec 14 11:50 dst
149194 -rw-r--r-- 1 roaima 29 Dec 14 11:50 dst~
137559 -rw-r--r-- 1 roaima 29 Dec 14 11:50 src

You can see that the destination (dst) file still has its original inode but that the backup has been given a new one. The second update shows that inodes are retained thereafter.

In your instance it does sound like --inplace --backup is what you need. However, please note that the man page warns,

WARNING: you should not use this option to update files that are being accessed by others, so be careful when choosing to use this for a copy.

The reason that --inplace is not a default is that an interrupted partial copy could leave the target file in an inconsistent state, and the author(s) decided that it would be better to have an internally consistent out-of-date file than a corrupted out-of-date file. Such behaviour can be modified with flags such as --inplace and --partial (and even --append with rsync versions 3.0.0 onwards).