bitcask failed to merge.merge_locked,locked

[ghost]

I have a riakcs-riak cluster and use the bitcask and leveldb as backend storage. But suddenly riak print these log in /var/log/riak/crash.log. Does anyone know how to fix it?
here is the log:

Failed to merge {["/mnt/riak/bitcask/182687704666362864775460604089535377456991567872/1359.bitcask.data","/mnt/riak/bitcask/182687704666362864775460604089535377456991567872/1358.bitcask.data","/mnt/riak/bitcask/182687704666362864775460604089535377456991567872/1357.bitcask.data","/mnt/riak/bitcask/182687704666362864775460604089535377456991567872/1354.bitcask.data","/mnt/riak/bitcask/182687704666362864775460604089535377456991567872/1353.bitcask.data","/mnt/riak/bitcask/182687704666362864775460604089535377456991567872/1244.bitcask.data","/mnt/riak/bitcask/182687704666362864775460604089535377456991567872/1121.bitcask.data","/mnt/riak/bitcask/182687704666362864775460604089535377456991567872/1061.bitcask.data"],[]}: {merge_locked,locked,"/mnt/riak/bitcask/18268770466636286