[sheepdog] no object found when 2 node killed in 10 nodes cluster with 3 copies

[redtone]

I create a sheepdog cluster with 10 nodes with 3 copies. 
When I killed 2 nodes in the cluster, some objects cannot be found.
In my mind, the cluster should be ok when 2 nodes is gone.

Sheepdog daemon version 0.7.9_rc0

After I kill node 4 and node 9 with dog node kill command 

I get the following node list.

[root at pangkvm10 ~]# dog node list
  Id   Host:Port         V-Nodes       Zone
   0   10.177.218.51:7000       109          1
   1   10.177.218.52:7000       31          1
   2   10.177.218.53:7000       61          1
   3   10.177.218.55:7000       54          1
   4   10.177.218.56:7000       108          1
   5   10.177.218.57:7000       34          1
   6   10.177.218.58:7000       92          1
   7   10.177.218.60:7000       23          1

[root at pangkvm10 ~]# dog vdi list
Name        Id    Size    Used  Shared    Creation time   VDI id  Copies
Tag
Failed to read object 803f4da200000000 No object found
Failed to read inode header
  kvm464_sheep     0  100 GB  1.7 GB  0.0 MB 2014-10-21 12:43   967a0f     3


Then is start node 9 again, the object is back.

[root at pangkvm10 ~]# dog vdi list
Name        Id    Size    Used  Shared    Creation time   VDI id  Copies
Tag
aaa          0   10 GB  0.0 MB  0.0 MB 2014-10-04 23:42   3f4da2     3

kvm464_sheep     0  100 GB  1.7 GB  0.0 MB 2014-10-21 12:43   967a0f     3