I think I found a bug affecting btrfs filesystems and users invoking fstrim to discard unused blocks: if I execute a `fstrim -v /` twice, the amount trimmed does not change on the 2nd invocation AND it takes just as long as the first. Why do I think this is a bug? When I do the same on an ext4 partition I get different behavior: the output shows 0 B trimmed and it does is instantaneously when I run it a 2nd time. After contacting the fstrim developer, he stated that the userspace part (fstrim) does only one thing and it is invoke an ioctl (FITRIM); it is the job of the filesystem to properly implement this.
Supporting data ---------------- Example on a btrfs partition: The 1st time: % time sudo fstrim -v / /: 5.2 GiB (5575192576 bytes) trimmed sudo fstrim -v / 0.00s user 0.05s system 2% cpu 2.084 total The 2nd time: % time sudo fstrim -v / /: 5.2 GiB (5575192576 bytes) trimmed sudo fstrim -v / 0.00s user 0.06s system 2% cpu 2.107 total If I run the command twice on an ext4 filesystem, it does go to zero and the 2nd invocation is instantaneous: The 1st time: % time sudo fstrim -v / /: 15.4 GiB (16481087488 bytes) trimmed sudo fstrim -v / 0.00s user 0.08s system 1% cpu 6.268 total The 2nd time: % time sudo fstrim -v / /: 0 B (0 bytes) trimmed sudo fstrim -v / 0.00s user 0.00s system 48% cpu 0.007 total -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
