How to factorize or share timemachine backups

[antoine23]

How to setup Timemachine (alternatively other backup app) to backup two macs + external storage in such a way that the external storage is backed-up once even if connected to any of the macs ?

The context, I have two macs setup the same (same OS, same apps), MacPro and MacbookPro.

I have an external drive with data, this allows me to access the data (for simplicity let's imagine it's all photos) from each mac alternatively.

I want to backup both macs plus the data on the external drive, to a second external drive, at whatever time both drives are plugged to a given mac.

I know I can remove the exclusion of external drives so that timemachine backup external drive, but I'm worried that it will backup the external drive twice, once when connected to the MacPro, and once when connected to the MacbookPro.

Has anybody tried something similar?

 

[Gregg2]

I'm probably missing something here, but my first thought is to do the TM backups manually rather than scheduled.

 

[antoine23]

Thanks, but I doubt that would change anything. My question relates to how to setup timemachine so that backing up external drive EXT when connected to Mac A will not be duplicated again when EXT is connected to Mac B and Mac B is backing up

MacA has drive A
MacB has drive B
EXT is an external drive

Ideally the backup drive should have:
MacA/A
MacB/B
EXT

I'm afraid it will have:
MacA/A
MacA/EXT
MacB/B
MacB/EXT

thus duplicating EXT

 

[chrfr]

Thanks, but I doubt that would change anything. My question relates to how to setup timemachine so that backing up external drive EXT when connected to Mac A will not be duplicated again when EXT is connected to Mac B and Mac B is backing up

MacA has drive A
MacB has drive B
EXT is an external drive

Ideally the backup drive should have:
MacA/A
MacB/B
EXT

I'm afraid it will have:
MacA/A
MacA/EXT
MacB/B
MacB/EXT

thus duplicating EXT

There’s no way to do this as Time Machine backups are separated by computer. The best you could do would be to exclude the backup of the external on one computer. That way the external disk will only be backed up whenever it’s connected to that one computer.

 

[BrianBaughn]

With Carbon Copy Cloner it seems like there would only be one backup of the first external on the second, backup external. In CCC on both computers that backup would be a separate task…but I've never tried what you're talking about and don't know what the CCC app would do when presented with this.

 

[Brian33]

With Carbon Copy Cloner it seems like there would only be one backup of the first external on the second, backup external. In CCC on both computers that backup would be a separate task…but I've never tried what you're talking about and don't know what the CCC app would do when presented with this.

Good idea!

Assuming no use of APFS snapshot support, I think the OP's scenario could work using CCC. It has the concept of Task but not Computer. The Task seems to be defined by the Volume UUID (by default) and not any "computer ID". So if the identical task were configured on the two Macs for the identical drives, it wouldn't matter that it was run on a different machine.

When NOT using snapshots, I think CCC just looks at every file and directory on the source volume and compares each "file signature" (file name, modification time, and size, by default) to the corresponding file on the destination volume. Again, it wouldn't matter which Mac was running the task, as long as the task settings were identical between the two Macs. I think it wouldn't even matter whether the drives were formatted APFS or HFS+, or even one of each, as long as APFS snapshot support was not being used for the CCC task.

(Actually, it probably would work with snapshots -- I just ruled that out to make the scenario simpler.)

This should be pretty easy to test, even with a trial version of Carbon Copy Cloner.

 

[frou]

Stepping back, it seems like it doesn't strictly matter if the backup system considers there to be 4 drives in total, what matters is that it internally does deduplication at the data level so that it doesn't waste space on the backup drive.

I don't know how comfortable you are using command-line software, but I'm almost certain Restic will handle this https://restic.net/