btrfs-progs: check: introduce low memory mode

Introduce a new fsck mode: low memory mode.

Old btrfsck is working efficiently but uses some memory for each extent
item.  This method will ensure extents are only iterated once at
extent/chunk tree check process.

But since it uses some memory for each extent item, for a large fs with
several TB metadata, this can easily eat up memory and cause OOM.

To handle such limitation and improve scalability, the new low-memory
mode will not use any heap memory to record which extent is checked.
Instead it will use extent backref to avoid most of uneeded checks on
shared fs/subvolume tree blocks.
And with the use forward and backward reference cross check, we can also
ensure every tree block is at least checked once.

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/Documentation/btrfs-check.asciidoc b/Documentation/btrfs-check.asciidoc
index 74a2ad21..4e27863e 100644
--- a/Documentation/btrfs-check.asciidoc
+++ b/Documentation/btrfs-check.asciidoc
@@ -93,6 +93,12 @@ build the extent tree from scratch
 +
 NOTE: Do not use unless you know what you're doing.
 
+--low-memory::
+check fs in low memory usage mode(experimental)
+May takes longer time than normal check.
++
+NOTE: Doesn't work with '--repair' option yet.
+
 EXIT STATUS
 -----------
 *btrfs check* returns a zero exit status if it succeeds. Non zero is