Btrfs: avoid tree log commit when there are no changes

rpm has a habit of running fdatasync when the file hasn't
changed.  We already detect if a file hasn't been changed
in the current transaction but it might have been sent to
the tree-log in this transaction and not changed since
the last call to fsync.

In this case, we want to avoid a tree log sync, which includes
a number of synchronous writes and barriers.  This commit
extends the existing tracking of the last transaction to change
a file to also track the last sub-transaction.

The end result is that rpm -ivh and -Uvh are roughly twice as fast,
and on par with ext3.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index ef399a7794f..5b9567caba0 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -3480,6 +3480,7 @@ static noinline void init_btrfs_i(struct inode *inode)
         bi->generation = 0;
         bi->sequence = 0;
         bi->last_trans = 0;
+        bi->last_sub_trans = 0;
         bi->logged_trans = 0;
         bi->delalloc_bytes = 0;
         bi->reserved_bytes = 0;
@@ -4980,7 +4981,9 @@ again:
         set_page_dirty(page);
         SetPageUptodate(page);
 
-        BTRFS_I(inode)->last_trans = root->fs_info->generation + 1;
+        BTRFS_I(inode)->last_trans = root->fs_info->generation;
+        BTRFS_I(inode)->last_sub_trans = BTRFS_I(inode)->root->log_transid;
+
         unlock_extent(io_tree, page_start, page_end, GFP_NOFS);
 
 out_unlock:
@@ -5100,6 +5103,7 @@ struct inode *btrfs_alloc_inode(struct super_block *sb)
         if (!ei)
                 return NULL;
         ei->last_trans = 0;
+        ei->last_sub_trans = 0;
         ei->logged_trans = 0;
         ei->outstanding_extents = 0;
         ei->reserved_extents = 0;