rbd: hook up cache options

Writeback caching was added in Ceph 0.46, and writethrough will be in
0.47. These are controlled by general config options, so there's no
need to check for librbd version.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/rbd.c

@@ -476,6 +476,25 @@ static int qemu_rbd_open(BlockDriverState *bs, const char *filename, int flags)
         s->snap = g_strdup(snap_buf);
     }
 
+    /*
+     * Fallback to more conservative semantics if setting cache
+     * options fails. Ignore errors from setting rbd_cache because the
+     * only possible error is that the option does not exist, and
+     * librbd defaults to no caching. If write through caching cannot
+     * be set up, fall back to no caching.
+     */
+    if (flags & BDRV_O_NOCACHE) {
+        rados_conf_set(s->cluster, "rbd_cache", "false");
+    } else {
+        rados_conf_set(s->cluster, "rbd_cache", "true");
+        if (!(flags & BDRV_O_CACHE_WB)) {
+            r = rados_conf_set(s->cluster, "rbd_cache_max_dirty", "0");
+            if (r < 0) {
+                rados_conf_set(s->cluster, "rbd_cache", "false");
+            }
+        }
+    }
+
     if (strstr(conf, "conf=") == NULL) {
         /* try default location, but ignore failure */
         rados_conf_read_file(s->cluster, NULL);