How data changes, and what changes it, matters when trying to optimize backups.
A full OS installation may not change a lot, or change with security updates that anyway are stored elsewhere.
Configurations have their own lifecycle, actors, and good practices on how to keep and backup them. Same with code.
Data is what matters if you have saved somewhat everything else. And it could have a different treatment file tree backups from I.e. database backups.
Logs is something that frequently changes, but you can have a proper log server for which logs are data.
Things can be this granular, or go for storage backup. But the granularity, while may add complexity, may lower costs and increase how much of what matters you can store for longer periods of time.
* Is the file userland-compressed, filesystem-or-device-compressed, or uncompressed?
* What are you going to do about secret keys?
* Is the file immutable, replace-only (most files), append-only (not limited to logs; beware the need to defrag these), or fully mutable (rare - mostly databases or dangerous archive software)?
* Can you rely on page size for (some) chunking, or do you need to rely entirely on content-based chunking?
* How exactly are you going to garbage-collect the data from no-longer-active backups?
* Does your filesystem expose an accurate "this file changed" signal, or better an actual hash? Does it support chunk sharing? Do you know how those APIs work?
* Are you crossing a kernel version that is one-way incompatible?
* Do you have control of the raw filesystem at the other side? (e.g. the most efficient backup for btrfs is only possible with this)
A full OS installation may not change a lot, or change with security updates that anyway are stored elsewhere.
Configurations have their own lifecycle, actors, and good practices on how to keep and backup them. Same with code.
Data is what matters if you have saved somewhat everything else. And it could have a different treatment file tree backups from I.e. database backups.
Logs is something that frequently changes, but you can have a proper log server for which logs are data.
Things can be this granular, or go for storage backup. But the granularity, while may add complexity, may lower costs and increase how much of what matters you can store for longer periods of time.