block: use fdatasync instead of fsync if possible

If we are flushing the caches for our image files we only care about the
data (including the metadata required for accessing it) but not things
like timestamp updates.  So try to use fdatasync instead of fsync to
implement the flush operations.

Unfortunately many operating systems still do not support fdatasync,
so we add a qemu_fdatasync wrapper that uses fdatasync if available
as per the _POSIX_SYNCHRONIZED_IO feature macro or fsync otherwise.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/cutils.c b/cutils.c
index ffe5c717e..7a2234646 100644
--- a/cutils.c
+++ b/cutils.c
@@ -115,6 +115,22 @@ int qemu_fls(int i)
     return 32 - clz32(i);
 }
 
+/*
+ * Make sure data goes on disk, but if possible do not bother to
+ * write out the inode just for timestamp updates.
+ *
+ * Unfortunately even in 2009 many operating systems do not support
+ * fdatasync and have to fall back to fsync.
+ */
+int qemu_fdatasync(int fd)
+{
+#ifdef _POSIX_SYNCHRONIZED_IO
+    return fdatasync(fd);
+#else
+    return fsync(fd);
+#endif
+}
+
 /* io vectors */
 
 void qemu_iovec_init(QEMUIOVector *qiov, int alloc_hint)