xfs: avoid a few disk cache flushes

There is no need for a pre-flush when doing writing the second part of a
split log buffer, and if we are using an external log there is no need
to do a full cache flush of the log device at all given that all writes
to it use the FUA flag.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

1 files changed, 12 insertions, 6 deletions

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 4c2d30e3b78d..0ef5eb677383 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -1371,15 +1371,21 @@ xlog_sync(xlog_t		*log,
         bp->b_flags |= XBF_SYNCIO;
 
         if (log->l_mp->m_flags & XFS_MOUNT_BARRIER) {
+                bp->b_flags |= XBF_FUA;
+
                 /*
-                 * If we have an external log device, flush the data device
-                 * before flushing the log to make sure all meta data
-                 * written back from the AIL actually made it to disk
-                 * before writing out the new log tail LSN in the log buffer.
+                 * Flush the data device before flushing the log to make
+                 * sure all meta data written back from the AIL actually made
+                 * it to disk before stamping the new log tail LSN into the
+                 * log buffer.  For an external log we need to issue the
+                 * flush explicitly, and unfortunately synchronously here;
+                 * for an internal log we can simply use the block layer
+                 * state machine for preflushes.
                  */
                 if (log->l_mp->m_logdev_targp != log->l_mp->m_ddev_targp)
                         xfs_blkdev_issue_flush(log->l_mp->m_ddev_targp);
-                bp->b_flags |= XBF_FUA | XBF_FLUSH;
+                else
+                        bp->b_flags |= XBF_FLUSH;
         }
 
         ASSERT(XFS_BUF_ADDR(bp) <= log->l_logBBsize-1);
@@ -1414,7 +1420,7 @@ xlog_sync(xlog_t		*log,
                 XFS_BUF_ASYNC(bp);
                 bp->b_flags |= XBF_SYNCIO;
                 if (log->l_mp->m_flags & XFS_MOUNT_BARRIER)
-                        bp->b_flags |= XBF_FUA | XBF_FLUSH;
+                        bp->b_flags |= XBF_FUA;
                 dptr = XFS_BUF_PTR(bp);
                 /*
                  * Bump the cycle numbers at the start of each block