Re: [PATCH RFC] Btrfs: add support for persistent mount options

[Eric Sandeen <sandeen@redhat.com>]

On 8/6/13 1:27 PM, Filipe David Borba Manana wrote:
> This change allows for most mount options to be persisted in
> the filesystem, and be applied when the filesystem is mounted.
> If the same options are specified at mount time, the persisted
> values for those options are ignored.

I thought the plan was to make commandline mount options
override persistent ones, but that doesn't seem to be the case,
at least in this example:

# ./btrfstune -o compress,discard,ssd /dev/sdb1
New persistent options:      compress,discard,ssd
# mount -o nossd /dev/sdb1 /mnt/test
# dmesg | tail
[  995.657233] btrfs: use ssd allocation scheme
[  995.661501] btrfs: disk space caching is enabled

and /proc/mounts is similarly confused, showing both options:

# grep sdb1 /proc/mounts
/dev/sdb1 /mnt/test btrfs rw,seclabel,relatime,compress=zlib,nossd,ssd,discard,space_cache 0 0

(This is the trail of woe I was talking about from ext4-land) ;)

> The only options not supported are: subvol, subvolid, subvolrootid,
> device and thread_pool. This limitation is due to how this feature
> is implemented: basically there's an optional value (of type
> struct btrfs_dir_item) in the tree of tree roots used to store the
> list of options in the same format as they are passed to btrfs_mount().
> This means any mount option that takes effect before the tree of tree
> roots is setup is not supported.

Just FWIW, vfs-level mount options are also not supported; i.e. "noatime"
or "ro" won't work either, because the code is already past the VFS
option parsing:

# ./btrfstune -o compress,discard,ro /dev/sdb1
Current persistent options:  compress,discard
New persistent options:      compress,discard,ro
# mount /dev/sdb1 /mnt/test
mount: wrong fs type, bad option, bad superblock on /dev/sdb1, ...
# dmesg | tail
[  817.681417] btrfs: unrecognized mount option 'ro'
[  817.694689] btrfs: open_ctree failed

> To set these options, the user space tool btrfstune was modified
> to persist the list of options into an unmounted filesystem's
> tree of tree roots.

If this goes forward, you'd want an easy way to display
them, not just set them, I suppose.

Thanks,
-Eric
 
> NOTE: Like the corresponding btrfs-progs patch, this is a WIP with
> the goal o gathering feedback.
> 
> Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>