I’ve been using Samba with the time machine setting for years to back up the couple of Macs I own. I’ve recently been running into issues with Time Machine backups erroring out with "The network backup disk does not support the required capabilities". Poking around the Internet didn’t really point at an obvious culprit until I found some mumblings about this potentially begin a disk space issue.

Bingo!

While the Samba server had more than enough disk space left, I had set the maximum Time Machine volume size to 3.5TB using the setting fruit:time machine max size = 3.5T. Bumping this up to 4.5T magically made Time Maching backups work immediatly.

macOS Time Machine is usually set up to work in the background and not overly affect anything that’s going on in the foreground while the user is working. Under normal circumstances, this is desirable behaviour. It is not desirable when you try to take one last backup of a failing SSD before it keels over completely. Which was the unfortunate situation I found myself in.

Turns out there is a sysctl that can be used to disable or enable this behaviour. If you turn it off, the backup in macOS Time Machine runs much faster, at the expense of additional network bandwidth and disk IOPS. The backup daemon will increase disk IOPS usage both for reading and writing.