xfs: make log devices with write back caches work

There's no reason not to support cache flushing on external log devices.
The only thing this really requires is flushing the data device first
both in fsync and log commits.  A side effect is that we also have to
remove the barrier write test during mount, which has been superflous
since the new FLUSH+FUA code anyway.  Also use the chance to flush the
RT subvolume write cache before the fsync commit, which is required
for correct semantics.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 211930246f20..41d5b8f2bf92 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -1372,8 +1372,17 @@ xlog_sync(xlog_t		*log,
         XFS_BUF_ASYNC(bp);
         bp->b_flags |= XBF_LOG_BUFFER;
 
-        if (log->l_mp->m_flags & XFS_MOUNT_BARRIER)
+        if (log->l_mp->m_flags & XFS_MOUNT_BARRIER) {
+                /*
+                 * 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.
+                 */
+                if (log->l_mp->m_logdev_targp != log->l_mp->m_ddev_targp)
+                        xfs_blkdev_issue_flush(log->l_mp->m_ddev_targp);
                 XFS_BUF_ORDERED(bp);
+        }
 
         ASSERT(XFS_BUF_ADDR(bp) <= log->l_logBBsize-1);
         ASSERT(XFS_BUF_ADDR(bp) + BTOBB(count) <= log->l_logBBsize);