Synchronized snapshot in synchronous replication

A non-transitory computer readable medium for time-aligning data sent from a first storage system (FSS) to a second storage system, the non-transitory computer readable medium stores instructions for:

• receiving multiple data units and a synchronization message, by a controller of the second storage system (SSS) and while applying a synchronous replication scheme:
• wherein the multiple data units comprise first snapshot (S1) data units and second snapshot (S2) data units;
• wherein the synchronization message indicates that a controller of the FSS has changed an allocation of data units from the S1 to the S2;
• wherein the receiving of the S1 data units, the S2 data units and the synchronization message is performed regardless of an order of transmitting the S1 data units, the S2 data units and the synchronization message from the FSS;
• and
• associating SSS timestamps with the multiple data units:
• wherein the associating of the SSS timestamps comprises:
• (c) associating a first SSS timestamp related to the S1 for each S1 data unit, regardless a timing order between a time of reception of the synchronization message and a time of reception of the S1 data unit, wherein a value of the first SSS timestamp does not exceed a value of a SSS clock upon reception of the synchronization message;
• and
• (d) delaying an association of a second SSS timestamp related to the S2 for each S2 data unit that is received before the synchronization message, and associating, after receiving the synchronization message, the second SSS timestamp to each S2 data unit whose association has been delayed, wherein a value of the second SSS timestamp is larger than the a value of the SSS clock upon reception of the synchronization message.