btrfs-progs: doc: update qgroup docs

Signed-off-by: David Sterba <dsterba@suse.com>

2 files changed, 18 insertions, 14 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.
diff --git a/cmds-qgroup.c b/cmds-qgroup.c
index 4a8b8d23..d58df5e1 100644
--- a/cmds-qgroup.c
+++ b/cmds-qgroup.c
@@ -206,7 +206,7 @@ static int parse_limit(const char *p, unsigned long long *s)
 
 static const char * const cmd_qgroup_assign_usage[] = {
 	"btrfs qgroup assign [options] <src> <dst> <path>",
-	"Enable subvolume qgroup support for a filesystem.",
+	"Assign SRC as the child qgroup of DST",
 	"",
 	"--rescan       schedule qutoa rescan if needed",
 	"--no-rescan    ",
@@ -223,7 +223,7 @@ static int cmd_qgroup_assign(int argc, char **argv)
 
 static const char * const cmd_qgroup_remove_usage[] = {
 	"btrfs qgroup remove <src> <dst> <path>",
-	"Remove a subvol from a quota group.",
+	"Remove a child qgroup SRC from DST.",
 	NULL
 };
 
@@ -251,7 +251,7 @@ static int cmd_qgroup_create(int argc, char **argv)
 
 static const char * const cmd_qgroup_destroy_usage[] = {
 	"btrfs qgroup destroy <qgroupid> <path>",
-	"Destroy a subvolume quota group.",
+	"Destroy a quota group.",
 	NULL
 };
 
@@ -419,7 +419,7 @@ static int cmd_qgroup_show(int argc, char **argv)
 
 static const char * const cmd_qgroup_limit_usage[] = {
 	"btrfs qgroup limit [options] <size>|none [<qgroupid>] <path>",
-	"Limit the size of a subvolume quota group.",
+	"Set the limits a subvolume quota group.",
 	"",
 	"-c   limit amount of data after compression. This is the default,",
 	"     it is currently not possible to turn off this option.",