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btrfs-progs: doc: Update btrfs-qgroup regarding the rescan conditions 

Add a new section, QUOTA RESCAN, to explain why and when we need a full
quota rescan when assigning/removing qgroup relationship.

Also, since 'remove' shares the same options of 'assign', add reference
to 'assign' options for 'remove' subcommand.

Signed-off-by: Qu Wenruo <wqu@suse.com>
[ update ]
Signed-off-by: David Sterba <dsterba@suse.com>
master
Qu Wenruo 2018-08-06 14:00:08 +08:00 committed by David Sterba
parent dda84b8a16
commit 325256a232
1 changed files with 24 additions and 2 deletions
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26
Documentation/btrfs-qgroup.asciidoc
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@ -48,9 +48,12 @@ identified by <path>.
+
--rescan::::
(default since: 4.19) Automatically schedule quota rescan if the new qgroup
assignment would lead to quota inconsistency.
assignment would lead to quota inconsistency. See 'QUOTA RESCAN' for more
information.
--no-rescan::::
Explicitly ask not to do a rescan.
Explicitly ask not to do a rescan, even if the assignment will make the quotas
inconsitent. This may be useful for repeated calls where the rescan would add
unnecessary overhead.
*create* <qgroupid> <path>::
Create a subvolume quota group.
@ -83,6 +86,10 @@ limit space exclusively assigned to this qgroup.
*remove* <src> <dst> <path>::
Remove the relationship between child qgroup <src> and parent qgroup <dst> in
the btrfs filesystem identified by <path>.
+
`Options`
+
The same as *assign* subcommand.
*show* [options] <path>::
Show all qgroups in the btrfs filesystem identified by <path>.
@ -131,6 +138,21 @@ If multiple <attr>s is given, use comma to separate.
To retrieve information after updating the state of qgroups,
force sync of the filesystem identified by <path> before getting information.
QUOTA RESCAN
------------
The rescan reads all extent sharing metadata and updates the respective qgoups
accordingly.
The information consists of bytes owned exclusively ('excl') or shared/referred
to ('rfer'). There's no explicit information about which extents are shared or
owned exclusively. This means when qgroup relationship changes, extent owners
change and qgroup numbers are no longer consistent unless we do a full rescan.
However there are cases where we can avoid a full rescan, if a subvolume whose
'rfer' number equals its 'excl' number, which means all bytes are exclusively
owned, then assigning/removing this subvolume only needs to add/subtract 'rfer'
number from its parent qgroup. This can speed up the rescan.
EXIT STATUS
-----------
*btrfs qgroup* returns a zero exit status if it succeeds. Non zero is