Operating System Software Owner manual

6. After the active cache update on the primary system is complete and the primary system
receives the backup system’s acknowledgement, Remote Copy sends an acknowledgement
of the write to the host.
Synchronization is complete.
Figure 36 Remote Copy in Synchronous Mode
Advantages of Synchronous Mode
• Even if the primary system, the backup system, or the communication links go down, Remote
Copy retains all I/O that has already been acknowledged.
• If a disaster affects the primary system and it goes down, the copy of the data that is stored
on the backup system is an exact replica of the data on the primary system and therefore can
immediately be used to continue working in the application.
Latency in Synchronous Mode
Synchronous mode adds latency to writes because:
• The I/O must be sent to the backup system over the IP network or FC SAN.
• The backup system must send the acknowledgement of successful replication over the IP network
or FC SAN before the host write is acknowledged.
As the distance between the primary and backup storage systems increases, the latency also
increases. For example, a one-way distance of 100 miles adds approximately two milliseconds to
the write latency.
Even with the primary and backup storage systems located side-by-side and linked by an FC SAN,
using synchronous mode volume groups adds some latency to a host write.
Asynchronous Periodic Mode
When Remote Copy volume groups operate in asynchronous periodic mode (Figure 37 (page 196)):
1. The host sends a write request only to the primary system.
2. Remote Copy creates snapshots of all virtual volumes within the volume group on the primary
system.
3. As soon as the data is written into cache on the primary system, Remote Copy acknowledges
the host write.
4. I/O resumes.
Asynchronous Periodic Mode 195