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Comment by pletnes | Hacker News Reader

Personally, I’ve never noticed bugs, but the performance characteristics are truly weird. If you fill up a large partition, expect using a few days of running various incantations to free space, even after removing plenty of files.

I’m completely amazed at how fast snapshots can be done, though.

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Yeah I'd classify that bad performance with nearly full drives as a bug. Compare that to one of my ext4 partitions which has been > 90% since years of active use without major issues. Occasionally I get errors that it can't write stuff any more due to capacity, I just delete some unneeded stuff and go on with life.

It's not really a bug though taken in context of how the file systems work with copy on write. You need somewhere to write, so if you low free space and have the drive heads seeking...

It really makes it seem like handling free space was an afterthought in btrfs.

Infinite memory and infinite storage are great abstractions, easy to use models and a lot of software runs on those models... but in this case it seems like ENOSPC should have been designed in from the start.

I have noticed lots of things break when the resource horizon shrinks. Weirdness happens as on crosses into 1/2 (for things that like to double, like amortized data structures) and then again at <5% of free space/cpu/network/etc. Often the best mitigation is to have excess reserve and remove the false blocker (file on disk, optional process eating ram, etc).

Just more anecdotes: I accidentally filled up a 2tb btrfs filesystem on ssd with a buggy backup script. I deleted the stuff that shouldn't be there, ran a few recommended incantations, and was back to normal in about ten minutes. It was much less of a big deal than I expected based on what I'd read.

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