FreeBSD had a pretty decent option in the base system two decades ago - FFS snapshots and a stock backup tool that would use them automatically with minimal effort, dump(8). Just chuck `-L` at it and your backups are consistent.
Now of course it's all about ZFS, so there's at least snapshots paired with replication - but the story for anything else is still pretty bad, with you having to put all the fiddly pieces together. I'm sure some people taught their backup tool about their special named backup snapshots sprinkled about in `.zfs/snapshot` directories, but given the fiddly nature of it I'm also sure most people just ended up YOLOing raw directories, temporal-smearing be damned.
I know I did!
I finally got around to fixing that last year with zfsnapr[1]. `zfsnapr mount /mnt/backup` and there's a snapshot of the system - all datasets, mounted recursively - ready for whatever backup tool of the year is.
I'm kind of disappointed in mentioning it over on the Practical ZFS forum that the response was not "why didn't you just use ", but "I can see why that might be useful".
Well, yes, it makes backups actually work.
> Also, it's unclear to me what happens if you attempt a snapshot in the middle of something like a database transaction or even a basic file write. Seems likely that the snapshot would still be corrupted
A snapshot is a point-in-time image of the filesystem at a given point. Any ACID database worth the name will roll back the in-flight transaction just like they would if you issued it a `kill -9`.
For other file writes, that's really down to whether or not such interruptions were considered by the writer. You may well have half-written files in your snapshot, with the file contents as they were in between two write() calls. Ideally this will only be in the form of temporary files, prior to their rename() over the data they're replacing.
For everything else - well, you have more than one snapshot backed up, right?
1: https://github.com/Freaky/zfsnapr