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>

4 files changed, 19 insertions, 2 deletions

diff --git a/block/cow.c b/block/cow.c
index 84818f1034..a70854e631 100644
--- a/block/cow.c
+++ b/block/cow.c
@@ -258,7 +258,7 @@ static int cow_create(const char *filename, QEMUOptionParameter *options)
 static void cow_flush(BlockDriverState *bs)
 {
     BDRVCowState *s = bs->opaque;
-    fsync(s->fd);
+    qemu_fdatasync(s->fd);
 }
 
 static QEMUOptionParameter cow_create_options[] = {
diff --git a/block/raw-posix.c b/block/raw-posix.c
index 2125d67a5f..2ebc88f004 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -723,7 +723,7 @@ static int raw_create(const char *filename, QEMUOptionParameter *options)
 static void raw_flush(BlockDriverState *bs)
 {
     BDRVRawState *s = bs->opaque;
-    fsync(s->fd);
+    qemu_fdatasync(s->fd);
 }
 
 
diff --git a/cutils.c b/cutils.c
index ffe5c717e4..7a2234646a 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)
diff --git a/qemu-common.h b/qemu-common.h
index f3cfb683ea..12e7dd0ada 100644
--- a/qemu-common.h
+++ b/qemu-common.h
@@ -114,6 +114,7 @@ int stristart(const char *str, const char *val, const char **ptr);
 int qemu_strnlen(const char *s, int max_len);
 time_t mktimegm(struct tm *tm);
 int qemu_fls(int i);
+int qemu_fdatasync(int fd);
 
 /* path.c */
 void init_paths(const char *prefix);