[libvirt] [PATCH] rbd: Use RBD fast-diff for querying actual volume allocation

[Wido den Hollander]

Since Ceph version Infernalis (9.2.0) the new fast-diff mechanism
of RBD allows for querying actual volume usage.

Prior to this version there was no easy and fast way to query how
much allocation a RBD volume had inside a Ceph cluster.

To use the fast-diff feature it needs to be enabled per RBD image
and is only supported by Ceph cluster running version Infernalis
(9.2.0) or newer.

Without the fast-diff feature enabled libvirt will report an allocation
identical to the image capacity. This is how libvirt behaves currently.

'virsh vol-info rbd/image2' might output for example:

  Name:           image2
  Type:           network
  Capacity:       1,00 GiB
  Allocation:     124,00 MiB

Newly created volumes will have the fast-diff feature enabled if the
backing Ceph cluster supports it.

Signed-off-by: Wido den Hollander <wido at widodh.nl>
---
 src/storage/storage_backend_rbd.c | 48 +++++++++++++++++++++++++++++++++++----
 1 file changed, 44 insertions(+), 4 deletions(-)

diff --git a/src/storage/storage_backend_rbd.c b/src/storage/storage_backend_rbd.c
index 5d73370..cdcfa48 100644
--- a/src/storage/storage_backend_rbd.c
+++ b/src/storage/storage_backend_rbd.c
@@ -279,6 +279,20 @@ virStorageBackendRBDCloseRADOSConn(virStorageBackendRBDStatePtr ptr)
     return ret;
 }
 
+#if LIBRBD_VERSION_CODE > 265
+static int
+virStorageBackendRBDRefreshVolInfoCb(uint64_t offset ATTRIBUTE_UNUSED,
+                                     size_t len,
+                                     int exists,
+                                     void *arg) {
+    uint64_t *used_size = (uint64_t *)(arg);
+    if (exists)
+        (*used_size) += len;
+
+    return 0;
+}
+#endif
+
 static int
 volStorageBackendRBDRefreshVolInfo(virStorageVolDefPtr vol,
                                    virStoragePoolObjPtr pool,
@@ -288,6 +302,8 @@ volStorageBackendRBDRefreshVolInfo(virStorageVolDefPtr vol,
     int r = 0;
     rbd_image_t image = NULL;
     rbd_image_info_t info;
+    uint64_t features ATTRIBUTE_UNUSED;
+    uint64_t used_size ATTRIBUTE_UNUSED = 0;
 
     if ((r = rbd_open_read_only(ptr->ioctx, vol->name, &image, NULL)) < 0) {
         ret = -r;
@@ -303,15 +319,39 @@ volStorageBackendRBDRefreshVolInfo(virStorageVolDefPtr vol,
         goto cleanup;
     }
 
-    VIR_DEBUG("Refreshed RBD image %s/%s (size: %zu obj_size: %zu num_objs: %zu)",
-              pool->def->source.name, vol->name, info.size, info.obj_size,
-              info.num_objs);
-
     vol->target.capacity = info.size;
     vol->target.allocation = info.obj_size * info.num_objs;
     vol->type = VIR_STORAGE_VOL_NETWORK;
     vol->target.format = VIR_STORAGE_FILE_RAW;
 
+#if LIBRBD_VERSION_CODE > 265
+    if ((r = rbd_get_features(image, &features)) < 0) {
+        virReportSystemError(-r, _("failed to get flags of RBD image '%s'"),
+                             vol->name);
+        goto cleanup;
+    }
+
+    if (features & RBD_FEATURE_FAST_DIFF) {
+        VIR_DEBUG("RBD image %s/%s has fast-diff enabled. Querying allocation",
+                  pool->def->source.name, vol->name);
+
+        if ((r = rbd_diff_iterate2(image, NULL, 0, info.size, 0, 1,
+                                   &virStorageBackendRBDRefreshVolInfoCb,
+                                   &used_size)) < 0) {
+            virReportSystemError(-r, _("failed to iterate RBD image '%s'"),
+                                 vol->name);
+            goto cleanup;
+        }
+
+        vol->target.allocation = used_size;
+    }
+#endif
+
+    VIR_DEBUG("Refreshed RBD image %s/%s (capacity: %llu allocation: %llu "
+                      "obj_size: %zu num_objs: %zu)",
+              pool->def->source.name, vol->name, vol->target.capacity,
+              vol->target.allocation, info.obj_size, info.num_objs);
+
     VIR_FREE(vol->target.path);
     if (virAsprintf(&vol->target.path, "%s/%s",
                     pool->def->source.name,
-- 
1.9.1