On 7/10/2011 4:21 AM, Arne Jansen wrote:
btrfs qgroup limit [--exclusive] <size>|none <qgroupid> <path>
This sets actual limits on a qgroup. If --exclusive is given, the
exclusive usage is limited instead of the referenced. I don't know
if there are use cases where both values need a (possibly different)
limit. <path> means the path to the root. Instead of "<qgroupid>
<path>", a path to a subvolume can be given instead.
btrfs qgroup create <qgroupid> <path>
btrfs qgroup destroy <qgroupid> <path>
btrfs qgroup assign <childid> <parentid> <path>
btrfs qgroup remove <childid> <parentid> <path>
These 4 commands are used to build hierarchical qgroups and are only
for advanced users. I'll explain more of the concepts in a later
paper.
The main point here is that in the simplest case, a user creates a
filesystem with initial quota support, creates his /var /usr /home
etc. subvolumes and limits them with commands like
btrfs qgroup limit 10g /usr
That should be simple enough for the common use case.
Wouldn't that make the syntax above actually be:
btrfs qgroup limit [--exclusive] <size|none> [qgroupid] <path>
Since the qgroupid is optional? And the meaning of path depends on
whether or not qgroupid is specified. With qgroupid, path is anywhere
on the fs, but without it, it specifies the path of the implicit
qgroupid, right?
I also have a question about the interactions with groups of groups.
Say I have 4 subvolumes: 1, 2, 3, and Z. I group the first 3 volumes
and set a limit on them. Now if all 3 volumes share a chunk of space,
that space should only count towards the group once, rather than 3
times. You might think the solution to that is to use the exclusive
limits, but that would mean that any space volume 3 and volume Z share
would not be counted in the group at all. I don't care about volume Z
since it is not part of the group, yet it can influence the used space
of the group. Likewise, if I set an exclusive limit on the group, and
then create snapshot Y from subvol 2, that would significantly reduce
the exclusive charge for the group, and we don't want that.
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