Re: What's the difference between `btrfs sub del -c` and `btrfs fi sync`?

[David Sterba <dsterba@suse.cz>]

On Sun, Apr 28, 2024 at 12:20:28PM +0200, intelfx@intelfx.name wrote:
> I see, thanks. That's what I suspected, but wanted to check if my
> (limited) understanding was correct.
> 
> It is unfortunate that ioctls that should be equivalent if judging by
> the names alone, in fact have subtle semantic differences that aren't
> documented anywhere.

Ioctls get documented incrementally, at least the FS SYNC is there,
https://btrfs.readthedocs.io/en/latest/btrfs-ioctl.html#btrfs-ioc-sync

> It's as if the ioctls were introduced specifically
> for the relevant userspace tools, encoding specific assumptions valid
> for their (only) consumers, without consideration of their wider
> applicability...

The start/wait ioctls can be used for cases when there's first phase
that generates some data or changes and second phase waits for the
commit. It was convenient to use that for the subvol sync, can be used
for other changes that eg. do only btrfs_end_transaction() in kernel
while some sort of sync semantics is wanted from user space (that would
be equivalent to btrfs_commit_transaction()).

You can read the discussions behind that in
https://www.spinics.net/lists/linux-btrfs/msg28354.html

The semantics of the start/wait ioctls were explained in the initial
patch https://git.kernel.org/linus/462045928bda777c86919a396a42991fcf235378
originally added for ceph.