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Comment by sp332 | Hacker News Reader

Since you have checkums available, you might as well do a periodic "scrub" and see if any of your data has been corrupted. Trim is useful for SSDs because it can make your drive's firmware more effective at wear leveling. Balance is pretty workload-specific and I don't have a heuristic for when it might make a difference. But being super unbalanced might cause performance issues.

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Scrubbing sounds useful. I'll start doing that every so often.

I thought trim was enabled by default (https://wiki.archlinux.org/title/Btrfs#SSD_TRIM). Does fstrim do something more than that or is it the same thing?

If there is no discard work to do, a fstrim on btrfs will do nothing. Discard=async, the new default, should be enough to trim deleted data without using fstrim. But nothing bad happens by using both.

IIUC the filesystem should balance itself. So unless you have added a new device to a mostly full filesystem you should be fine.

If you delete a bunch of files it might get unbalanced? That wouldn't degrade performance though I think. Or it is one of those FS's that makes sure everything is balanced on disk anyway?

In theory yes. But it is unlikely. The files are very likely balanced before you delete them so it should be mostly balanced after. It will also self-correct the difference after a bit more use.

Unbalanced drives can affect performance as one drive will have to do more of the work often bottlenecking. But except for extreme cases it is probably a rounding error.

I don't understand the details but I know one case it can fix problems is after deleting files to free up space on a nearly full disk.

Btrfs has separate allocation pools for data and metadata. If you delete files, the freed-up space is returned to the data pool but that does not make it generally-available for use in the metadata pool. So in the case where the entire disk space is already allocated to one of the allocation pools, none of the pools can grow beyond their current size.

One of the effects of balancing the tree is to release unused space from the allocation pools to general availability, solving that problem.

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