tree ad047fe5796156aa88e3f3600111bf2b8d12759f
parent 4722607db6a78bd7748c51fa4c8d7371da797254
author Chris Mason <chris.mason@oracle.com> 1255454468 -0400
committer Chris Mason <chris.mason@oracle.com> 1255455312 -0400

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>