RSYNC is a powerful file and directory synchronization tool widely used in Unix and Linux operating systems for data backup, mirroring, and file transfer. Sometimes, it’s required for RSYNC to copy all files again, even if they seem to be identical. This may be necessary if you suspect corruption in already synchronized files or during debugging issues.
By default, RSYNC uses a quick and efficient algorithm to determine if a file needs synchronization: it compares the file size and the modification times between the source and the destination files. If they match, RSYNC assumes that the file has not changed and will not re-copy it. However, you can override this behavior using certain RSYNC options.
The simple way to force RSYNC to copy all files again is to use the ‘—ignore-times’ or the shorthand ‘-I’ option.
Here is an example of how the ‘—ignore-times’ option would be used in a command:
```
rsync -avI /source/directory/ /destination/directory/
```
In this command, RSYNC would compare and copy all files from the source to the destination, regardless of whether they seem to be identical or not.
Please note, using ‘—ignore-times’ can make RSYNC significantly slower, especially for large datasets, because it causes RSYNC to check every single file in the dataset. Additionally, this will result in unnecessary data transfer if the files have not changed, which may not be ideal in all situations.
Another thing to keep in mind is that even with ‘—ignore-times’, RSYNC will not re-copy data that it previously synchronized if the data has not changed. This is due to its use of checksums to compare file contents. If you need to force a complete re-copy of every file, i.e., to create entirely new files rather than updating the existing ones, you must also use ‘—delete’ option.
```
rsync —avI —delete /source/directory/ /destination/directory/
```
Keep in mind that the ‘—delete’ option will delete everything in the destination that doesn’t exist in the source, so be careful with this command.
Sources:
1. Rsync man page: https://linux.die.net/man/1/rsync
2. How to force rsync to overwrite destination files, irrespective of timestamps: https://stackoverflow.com/questions/10382141/how-to-force-rsync-to-overwrite-destination-files-irrespective-of-timestamps
3. Rsync: https://wiki.archlinux.org/title/rsync
4. Using Rsync and SSH: https://www.hypexr.org/linux_rsync_and\_ssh.php
