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Diffstat (limited to 'Documentation/btrfs-qgroup.asciidoc')
| -rw-r--r-- | Documentation/btrfs-qgroup.asciidoc | 24 |
1 files changed, 14 insertions, 10 deletions
diff --git a/Documentation/btrfs-qgroup.asciidoc b/Documentation/btrfs-qgroup.asciidoc index cd7192c0..57cf012d 100644 --- a/Documentation/btrfs-qgroup.asciidoc +++ b/Documentation/btrfs-qgroup.asciidoc @@ -11,25 +11,29 @@ SYNOPSIS DESCRIPTION ----------- -*btrfs qgroup* is used to control quota group(qgroup) of a btrfs filesystem. +*btrfs qgroup* is used to control quota group (qgroup) of a btrfs filesystem. -NOTE: To use qgroup, it needs to enable quota first using *btrfs quota* +NOTE: To use qgroup you need to enable quota first using *btrfs quota enable* command. WARNING: Qgroup is not stable yet and will impact performance in current mainline -kernel(v3.14 so far). +kernel (v3.14 so far). QGROUP ------ -Quota group or qgroup in btrfs has its hierarchy like subvolume. -One subvolume/snapshot can reach its quota limits if it consumes all the quota -assigned to it or any of the parent qgroup(s). +Quota groups or qgroup in btrfs make a tree hierarchy, the leaf qgroups are +attached to subvolumes. The size limits are set per qgroup and apply when any +limit is reached in tree that contains a given subvolume. -Also for snapshot, it consumes no quota initially since all its data -shares with its parent, so only modification in snapshot consumes quota. +The limit sare separated between shared and exclusive and reflect the extent +ownership. For example a fresh snapshot shares almost all the blocks with the +original subvolume, new writes to either subvolume will raise towards the +exclusive limit. -Every subvolume/snapshot will have its own qgroup with id '0/<subvolume id>' -upon creating, but can be later destroyed by *btrfs qgroup destroy* command. +The qgroup identifiers conform to 'level/id' where level 0 is reserved to the +qgroups associated with subvolumes. Such qgroups are created automatically. + +The qgroup hierarchy is built by commands *create* and *assign*. NOTE: If the qgroup of a subvolume is destroyed, quota about the subvolume will not be functional until qgroup '0/<subvolume id>' is created again. |