[PATCH 3/8] btrfs: don't use global rsv for chunk allocation

[Josef Bacik <josef@toxicpanda.com>]

The should_alloc_chunk code has math in it to decide if we're getting
short on space and if we should go ahead and pre-emptively allocate a
new chunk.  Previously when we did not have the delayed_refs_rsv, we had
to assume that the global block rsv was essentially used space and could
be allocated completely at any time, so we counted this space as "used"
when determining if we had enough slack space in our current space_info.
But on any slightly used file system (10gib or more) you can have a
global reserve of 512mib.  With our default chunk size being 1gib that
means we just assume half of the block group is used, which could result
in us allocating more metadata chunks than is actually required.

With the delayed refs rsv we can flush delayed refs as the space becomes
tight, and if we actually need more block groups then they will be
allocated based on space pressure.  We no longer require assuming the
global reserve is used space in our calculations.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/btrfs/extent-tree.c | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 204b35434056..667b992d322d 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -4398,21 +4398,12 @@ static inline u64 calc_global_rsv_need_space(struct btrfs_block_rsv *global)
 static int should_alloc_chunk(struct btrfs_fs_info *fs_info,
 			      struct btrfs_space_info *sinfo, int force)
 {
-	struct btrfs_block_rsv *global_rsv = &fs_info->global_block_rsv;
 	u64 bytes_used = btrfs_space_info_used(sinfo, false);
 	u64 thresh;
 
 	if (force == CHUNK_ALLOC_FORCE)
 		return 1;
 
-	/*
-	 * We need to take into account the global rsv because for all intents
-	 * and purposes it's used space.  Don't worry about locking the
-	 * global_rsv, it doesn't change except when the transaction commits.
-	 */
-	if (sinfo->flags & BTRFS_BLOCK_GROUP_METADATA)
-		bytes_used += calc_global_rsv_need_space(global_rsv);
-
 	/*
 	 * in limited mode, we want to have some free space up to
 	 * about 1% of the FS size.
-- 
2.14.3