Other things that matter (some overlap): * Is the file userland-compressed, filesystem-or-device-compressed, or uncompressed? * What are you going to do about

Other things that matter (some overlap): * 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)