block: add cache=directsync parameter to -drive

This patch adds -drive cache=directsync for O_DIRECT | O_SYNC host file
I/O with no disk write cache presented to the guest.

This mode is useful when guests may not be sending flushes when
appropriate and therefore leave data at risk in case of power failure.
When cache=directsync is used, write operations are only completed to
the guest when data is safely on disk.

This new mode is like cache=writethrough but it bypasses the host page
cache.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/block.c b/block.c
index dbef3ae672..4186a2f340 100644
--- a/block.c
+++ b/block.c
@@ -448,6 +448,8 @@ int bdrv_parse_cache_flags(const char *mode, int *flags)
 
     if (!strcmp(mode, "off") || !strcmp(mode, "none")) {
         *flags |= BDRV_O_NOCACHE | BDRV_O_CACHE_WB;
+    } else if (!strcmp(mode, "directsync")) {
+        *flags |= BDRV_O_NOCACHE;
     } else if (!strcmp(mode, "writeback")) {
         *flags |= BDRV_O_CACHE_WB;
     } else if (!strcmp(mode, "unsafe")) {
@@ -1188,8 +1190,8 @@ int bdrv_pwrite_sync(BlockDriverState *bs, int64_t offset,
         return ret;
     }
 
-    /* No flush needed for cache=writethrough, it uses O_DSYNC */
-    if ((bs->open_flags & BDRV_O_CACHE_MASK) != 0) {
+    /* No flush needed for cache modes that use O_DSYNC */
+    if ((bs->open_flags & BDRV_O_CACHE_WB) != 0) {
         bdrv_flush(bs);
     }