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

Christoph Hellwig
2011-06-29T14:03:43Z

[-- Attachment #1: xfs-avoid-cache-flushes --]
[-- Type: text/plain, Size: 1999 bytes --]

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>

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