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Typo, I meant tolerates. Fixed it.

Not an option either, because if I reboot two machines and the backup starts in both of them it'll cripple my NAS

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How does systemd on the 2 machines avoid that? Are they communicating somehow?
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No, just different cron schedules. If I just reboot a machine the job doesn't get triggered, only if I start a machine after the cron schedule should have been triggered. To be fair, if I start two machines in these conditions this will happen too, but such situation is much more manageable than rebooting too machines in a short period of time.
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"tolerates".
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