Btrfs: improve performance on fsync of files with multiple hardlinks

Commit 41bd6067692382 ("Btrfs: fix fsync of files with multiple hard links
in new directories") introduced a path that makes fsync fallback to a full
transaction commit in order to avoid losing hard links and new ancestors
of the fsynced inode. That path is triggered only when the inode has more
than one hard link and either has a new hard link created in the current
transaction or the inode was evicted and reloaded in the current
transaction.

That path ends up getting triggered very often (hundreds of times) during
the course of pgbench benchmarks, resulting in performance drops of about
20%.

This change restores the performance by not triggering the full transaction
commit in those cases, and instead iterate the fs/subvolume tree in search
of all possible new ancestors, for all hard links, to log them.

Reported-by: Zhao Yuhu <zyuhu@suse.com>
Tested-by: James Wang <jnwang@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 0 insertions, 6 deletions

diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
index b16c13d51be0..d5b438706b77 100644
--- a/fs/btrfs/btrfs_inode.h
+++ b/fs/btrfs/btrfs_inode.h
@@ -148,12 +148,6 @@ struct btrfs_inode {
 	u64 last_unlink_trans;
 
 	/*
-	 * Track the transaction id of the last transaction used to create a
-	 * hard link for the inode. This is used by the log tree (fsync).
-	 */
-	u64 last_link_trans;
-
-	/*
 	 * Number of bytes outstanding that are going to need csums.  This is
 	 * used in ENOSPC accounting.
 	 */