--inplace

This option changes how rsync transfers a file when the file's data needs to be updated: instead of the default method of creating a new copy of the file and moving it into place when it is complete, rsync instead writes the updated data directly to the destination file.

This has several effects: (1) in-use binaries cannot be updated (either the OS will prevent this from happening, or binaries that attempt to swap-in their data will misbehave or crash), (2) the file’s data will be in an inconsistent state during the transfer, (3) a file’s data may be left in an inconsistent state after the transfer if the transfer is interrupted or if an update fails, (4) a file that does not have write permissions can not be updated, and (5) the efficiency of rsync’s delta-transfer algorithm may be reduced if some data in the destination file is overwritten before it can be copied to a position later in the file (one exception to this is if you combine this option with –backup, since rsync is smart enough to use the backup file as the basis file for the transfer).
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. This option is useful for transfer of large files with block-based changes or appended data, and also on systems that are disk bound, not network bound. The option implies –partial (since an interrupted transfer does not delete the file), but conflicts with –partial-dir and –delay-updates. Prior to rsync 2.6.4 –inplace was also incompatible with –compare-dest and –link-dest.