alt Hacker News> "Preparing backup..." taking an unreasonable amount of time is a regular occurrence,
TM heavily throttles disk I/O used for backing up in order to ensure that normal user activity isn't affected. That makes it appear that TM is dramatically slower than you would expect which greatly annoys me. This becomes obvious after you run this command which will make both the preparing and transferring phases go closer to the theoretical speed you'd expect:
sudo sysctl debug.lowpri_throttle_enabled=0
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> TM heavily throttles disk I/O used for backing up
That makes sense, and I usually quite like that behavior. I barely ever notice an impact when backups are running.
However, this is happening every time on one machine (Intel iMac), and semi-regularly on another one (M3 MBP), after a fresh restart, giving mds_stores some time to settle down, and the most recent backup just hours ago, with no significant changes on disk since.
In a situation like that, I would expect the "Preparing backup..." stage to just take a second to create an APFS snapshot, and maybe a minute to diff that snapshot against the remote state. But not 10+ minutes.
But thank you for the hint about that sysctl parameter! I will certainly give this a try.
TM makes my M2 Air chug when it runs against a local Samsung SSD. It does no such I/O throttling.