etcdserver/storage.go
func (st *storage) SaveSnap(snap raftpb.Snapshot) error {
	walsnap := walpb.Snapshot{
		Index: snap.Metadata.Index,
		Term:  snap.Metadata.Term,
	}
	err := st.WAL.SaveSnapshot(walsnap)
	if err != nil {
		return err
	}
	err = st.Snapshotter.SaveSnap(snap)
	if err != nil {
		return err
	}
	return st.WAL.ReleaseLockTo(snap.Metadata.Index)
}
func (st *storage) SaveSnap(snap raftpb.Snapshot) error {
	walsnap := walpb.Snapshot{
		Index: snap.Metadata.Index,
		Term:  snap.Metadata.Term,
	}
	err := st.WAL.SaveSnapshot(walsnap)
	if err != nil {
		return err
	}
	err = st.Snapshotter.SaveSnap(snap)
	if err != nil {
		return err
	}
	return st.WAL.ReleaseLockTo(snap.Metadata.Index)
}
// SaveSnap saves the snapshot to disk and release the locked
// wal files since they will not be used.