Re: [RFC PATCHv2 0/3] Btrfs-progs: introduce btrfs property subgroup

On Wed, Jun 27, 2012 at 7:41 PM, Goffredo Baroncelli [off-list ref] wrote:

Hi Alexander,

On 06/27/2012 03:16 PM, Alexander Block wrote:

quoted

This patchset introduces the btrfs property subgroup. It is the
result of a discussion we had on IRC. I tried to make the properties
interface as generic and extensible as possible. Comments are welcome.

Currently the command group looks like this:
btrfs prop set [-t <type>] /path/to/object <name> <value>
btrfs prop get [-t <type>] /path/to/object [<name>] (omitting name dumps all)
btrfs prop list [-t <type>] /path/to/object (lists properties with description)

The type is used to explicitly specify what type of object you mean. This is
necessary in case the object+property combination is ambiguous.  For example
'/path/to/fs/root' could mean the root subvolume, the directory inode or the
filesystem itself. Normally, btrfs-progs will try to detect the type
automatically.

Instead of using

  btrfs prop set -t filesystem ....
  btrfs prop set -t subvolume ....
  btrfs prop set -t inode ....

what about

  btrfs filesystem prop set .....
  btrfs subvolume prop set .....
  btrfs inode prop set .....

?

Specifying the object type is not needed normally, so the form where
the prop commands
are in their own command group is normally shorter and easier to type.
The type argument
is really only needed for ambiguous properties.

btrfs has already grouped the commands by "category" (even tough I have
to admit that some command doesn't fit its group. Why do not use this
instead of "-t <object type>". If we need new type we could create it on
demand...

quoted

David suggested that it should also be possible to specify objects by
their id/uuid/fsid. I like that idea, but would be happy if someone else
could take over that part :)

This proposal makes sense.

quoted

For now, I've implemented two properties:
1. read-only. Usable on subvolumes to toggle the read-only flags.
2. label. I looked through btrfs to find good examples of things that
   could be moved to the new properties interface and the filesystem
   label looked like a good one. There are for sure more, but that is
   something for later (and maybe for someone else). I would suggest
   to move everthing that makes sense over to the props interface and
   mark the old interfaces as deprecated. Comments on this are welcome.

What is the suppose goal/benefit/advantage to switch to a new interface
? Really, it is a true question.

The reason is that there will come a lot more things that you can
configure on an
inode/subvolume/filesystem/device basis and we agreed that a generic interface
for such configurations will be a good thing in the future.

quoted

Patch version history:
v1
  Initial version.
v2
  - Removed the filesystem prefix and implemented it as new command group
  - Switched from the <name>[=<value>] form to the set/get <name> [<value>]
    form.
  - Removed patches "Btrfs-progs: make filesystem_cmd_group non const"
    and "Btrfs-progs: move skip_prefix and prefixcmp to utils.c". They
    are not needed anymore due to the 'btrfs prop list' command.
  - Udjusted the subvol flags patch to be compatible to the "Btrfs: use
    _IOR for BTRFS_IOC_SUBVOL_GETFLAGS" patch.
  - Using -t <type> instead of <type>: prefix now.
  - Changes are based on feedback from Ilya and David.

Alex.

Alexander Block (3):
  Btrfs-progs: add BTRFS_IOC_SUBVOL_GET/SETFLAGS to ioctl.h
  Btrfs-progs: let get_label return the label instead of of printing it
  Btrfs-progs: introduce btrfs property subgroup

 Makefile          |    5 +-
 btrfs.c           |    1 +
 btrfslabel.c      |   13 +-
 btrfslabel.h      |    4 +-
 cmds-filesystem.c |   14 +-
 cmds-property.c   |  459 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 commands.h        |    2 +
 ioctl.h           |    2 +
 props.c           |  114 +++++++++++++
 props.h           |   43 +++++

Please update the man page too... This could help the process of
creating a new interface.

quoted

 10 files changed, 645 insertions(+), 12 deletions(-)
 create mode 100644 cmds-property.c
 create mode 100644 props.c
 create mode 100644 props.h