HP XP7 Storage - Manual (Page 38)

2. From the spreadsheet or graph, locate the largest or highest rolling average value. This is the
peak rolling average, which indicates the base amount of data that your bandwidth must be
able to handle.
3. With a base bandwidth value established, make adjustments for growth and a safety factor.
•
Projected growth rate accounts for the increase expected in write-workload over a 1, 2,
or 3 year period.
•
A safety factor adds extra bandwidth for unusually high spikes that did not occur during
write-workload measurement but could.
Other factors that must be taken into consideration because of their effect on bandwidth are
latency and packet loss. These are discussed in the following topics.
Latency
Network latency affects replication. It is the amount of data that can be present in the data path.
In the event of network failure, a certain number of transmitted records will not yet be resident in
the secondary system’s journal because they are still in-route within the data path. During periods
of low workload, there may be no records in the path, but during periods of heavy workload, the
network is fully used. This amount represents the minimum difference between data in the primary
and secondary systems.
Packet loss
Packet losses have the effect of reducing overall bandwidth because lost packets must be
re-transmitted, which consumes network capacity that would otherwise be occupied by new data
traffic. Also, a network can elongate consistency time, since journals are not applied until a
contiguous sequence of records has arrived at the remote site.
Planning ports for data transfer
When new data exists in the P-VOL, informational commands are sent from the primary system
initiator port to the secondary system RCU target port.
38 Planning the data path