Virtual　optical　networks　(VONs)　have　been　considered　as　a　promising　solution　to　support　current　high-capacity　dynamic　traffic　and　achieve　rapid　applications　deployment.　Since　most　of　the　network　services　(e.g.,　high-definition　video　service,　cloud　computing,　distributed　storage)　in　VONs　are　provisioned　by　dedicated　data　centers,　needing　different　amount　of　bandwidth　resources　in　both　directions,　the　network　traffic　is　mostly　asymmetric.　The　common　strategy,　symmetric　provisioning　of　traffic　in　optical　networks,　leads　to　a　waste　of　spectrum　resources　in　such　traffic　patterns.　In　this　paper,　we　design　a　holding-time-aware　asymmetric　spectrum　allocation　module　based　on　SDON　architecture　and　an　asymmetric　spectrum　allocation　algorithm　based　on　the　module　is　proposed.　For　the　purpose　of　reducing　spectrum　resources＇　waste,　the　algorithm　attempts　to　reallocate　the　idle　unidirectional　spectrum　slots　in　VONs,　which　are　generated　due　to　the　asymmetry　of　services＇　bidirectional　bandwidth.　This　part　of　resources　can　be　exploited　by　other　requests,　such　as　short-time　non-VON　requests.　We　also　introduce　a　two-dimensional　asymmetric　resource　model　for　maintaining　idle　spectrum　resources　information　of　VON　in　spectrum　and　time　domains.　Moreover,　a　simulation　is　designed　to　evaluate　the　performance　of　the　proposed　algorithm,　and　results　show　that　our　proposed　asymmetric　spectrum　allocation　algorithm　can　improve　the　resource　waste　and　reduce　blocking　probability.