When inheriting a backup with a new machine, will Time Machine deduplicate my data?

Question

I have an APFS-formatted drive with a year or so of Time Machine backup from my old machine. I now got a new Mac. When setting it up, I did not migrate any system files or users, instead copying user files manually for a fresh start. So in Time Machine's eyes it's a brand-new machine. Now I should, of course, be able to inherit/associate the old backups to the new machine (as plenty of threads on this site discuss).

I was wondering, though, when told to inherit, will Time Machine deduplicate its data, i.e., automatically detect files that are the same and only use new space for those that have changed, or will it create an entirely new copy of all data? Because if the latter is true, then what's the point of inheriting? I could just create a new APFS volume to back up my new Mac and leave the other volume untouched.

In other words, will I save any space on the backup drive if I inherit the old Time Machine backups? Or are there any other possible benefits?

Answer 1

You shouldn’t have to do anything as macOS wants to discover and ask you about inheriting.

• https://support.apple.com/en-us/104984

Your Mac might ask whether this computer should claim existing backups created on a different computer. Claim the backups if you want them to become part of the backups for this Mac.

When things work, you have one unified backup and no duplication of data from the last backup on the old machine and the next backup on the new machine. If you discover otherwise, we can help with specific details you provide. If files move, expect space to not get reclaimed by Time Machine and the only benefit is you have one machine to look through for all the time intervals instead of two machines on the backup with disjoint timelines. It would be the same situation if you took one backup drive to two Macs for those time periods. Either could restore files from either "backup".

• jan: I'm still curious, does that imply that "when things work" TM will do data deduplication, even if it's a fully different machine? I know in the past with HSF+ volumes that didn't work – or one had to manipulate the UUIDs to make sure backups are associated – but even that wouldn't always ensure TM continued with incremental backups starting anew with a full one. I would hope things have improved now with APFS but I couldn't find any documentation.
• bmike: You need a different tool (like Hyperspace)to detect and deduplicate data by using APFS file clones within a volume (or hard links for non-APFS-based space optimizations). What Time Machine does is hard link and APFS clones of files that are unchanged (path and content) between intervals. That’s what I hoped to convey by "when things work" means.
• jan: Thanks for pointing to Hyperspace, didn't know that existed – will come in handy, although for a different scenario. (I assume that it won't be able to manipulate TM backups after the fact?) In any case, I'm still wondering if TM will hard link to existing files in my scenario without copying them over (given it's a new computer and files have moved). – I've clarified my question a bit more.
• bmike: I would be very surprised if you could save space in that case expecting Time Machine to do it. See my edit to add path and content to my comment suggesting Hyperspace