HP 3PAR Operating System Software - Manual (Page 121)

6. After the active cache update on the primary system is complete and the primary system
receives the backup system’s acknowledgment, the primary array sends an acknowledgment
of the write to the host.
I/O to the host 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, both
storage systems contain all the data for I/O that has already been acknowledged to the server.
•
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
The synchronous mode adds latency to host write I/O because:
•
The I/O must be sent to the backup system over the IP network or FC SAN, which adds latency
to the write I/O response time.
•
The backup system must process the host write I/O before the host write is acknowledged
back to the primary system.
As the distance between the primary and backup storage systems increases, the latency also
increases. For example, a one-way distance of 100 miles over dark fiber adds approximately 1
ms to the write latency.
In synchronous mode, host writes must be sent to the backup system, and an acknowledgment must
be sent back to the primary system, before the host is acknowledged. This process adds latency
to host writes.
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 (see Figure 37
(page 122) ):
Volume Group Modes 121