[PATCH 27/27] xfs: avoid a few disk cache flushes

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

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@xxxxxx>

Index: xfs/fs/xfs/xfs_log.c
===================================================================
--- xfs.orig/fs/xfs/xfs_log.c   2011-06-29 14:27:59.166808258 +0200
+++ xfs/fs/xfs/xfs_log.c        2011-06-29 14:30:22.779363576 +0200
@@ -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