The Apple support article “About Time Machine local snapshots” says that “if less than 20% of the total storage space on your startup drive is available, Time Machine removes local snapshots, starting with the oldest, until you have more than 20% free space.”

If I understand that correctly, that means the local snapshots feature stops working as soon as you fill up 80% of your drive. For a 500GB drive, Time Machine insists on keeping at least 100GB free.

That seems like a lot of space to keep free. I’m not sure why that would be necessary. As a side question: is there any reason why it is specified as a relative percentage of the drive’s size rather than as an absolute number?

Main question: is there any way to decrease this from 20% to, for example, just 10% (so 50GB for a 500GB drive)?

  • Why percentage? Because 50 GB (or 100 GB) out of a 128 GB SSD is a lot more than 20% of that drive. – tubedogg Jun 18 '15 at 19:38
  • Yes. But look at it the other way around. On a 64 GB drive (like in the original 11" MacBook Air), Time Machine will happily keep local snapshots until you only have 12.8 GB left. Why isn’t it also 12.8 GB for a 512GB drive then? Why does it have to keep 8 times as much space free? – Rinzwind Jun 18 '15 at 20:02
  • I would argue it shouldn't be eating space down to 12.8 GB - the more space available, the happier OS X is. In any event, they had to choose percentage or absolute, and for whatever reason they chose percentage. – tubedogg Jun 18 '15 at 20:09
  • It could also have been something like “20% of total space or 50GB, whichever is smaller”. It seems odd that on a MacBook Pro with a 1000GB drive you need to keep at least 200GB free to get any benefit from the local snapshot feature. One wonders what all that free space needs to be kept for. Perhaps someone else has an explanation. – Rinzwind Jun 18 '15 at 20:28

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