[Qemu-devel] [PATCH] rbd: hook up cache options

[Josh Durgin <josh.durgin@inktank.com>]

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>
---
 block/rbd.c |   19 +++++++++++++++++++
 1 files changed, 19 insertions(+), 0 deletions(-)

diff --git a/block/rbd.c b/block/rbd.c
index 1280d66..eebc334 100644
--- a/block/rbd.c
+++ b/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);
-- 
1.7.5.4