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>
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index ac8927b..d4132aa 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -919,6 +919,7 @@
atomic_set(&root->log_writers, 0);
root->log_batch = 0;
root->log_transid = 0;
+ root->last_log_commit = 0;
extent_io_tree_init(&root->dirty_log_pages,
fs_info->btree_inode->i_mapping, GFP_NOFS);
@@ -1089,6 +1090,7 @@
WARN_ON(root->log_root);
root->log_root = log_root;
root->log_transid = 0;
+ root->last_log_commit = 0;
return 0;
}