HP XP Continuous Access Software - Manual (Page 63)

where:
• V
H-M
is the data transfer speed between the hosts and primary array
• V
M-R
is the data transfer speed between the primary and secondary arrays
• t is the length of time during which data transfer can continue
To calculate the journal volume capacity required when a communications path failure occurs between
the primary and secondary arrays, assign 0 (zero) to V
M-R
.
The total capacity of the journal volumes in each journal group must exceed the value illustrated previously.
Journal volumes in restore journal groups
When a failure occurs at a host, one failure recovery measure is to reverse the primary and secondary
data volumes (that is, change the copy direction). To reverse the primary and secondary data volumes,
you must usually ensure that the data transfer speed is the same before and after reversing the data
volumes. Therefore, when planning for master and restore journal volumes, you must apply the same
scheme to the master and restore journal volumes.
If you do not want to reverse the primary and secondary data volumes, you can cope with a temporary
increase in data transfers and a communications path failure between the primary and secondary arrays
if the master journal volume satisfies the conditions mentioned previously. Therefore, the data transfer
speed and volume capacity required for restore journal volumes are smaller than those required for
master journal volumes. The data transfer speed for journal volumes must exceed the data transfer
speed in “normal status.”
Factors that determine the data transfer speed between the primary and
secondary arrays
The data transfer speed between the primary and secondary arrays depends on the following factors:
• Bandwidth of the data transfer paths
• DKC's journal transfer speed
The data transfer speed between the primary and secondary arrays must exceed the data transfer speed
in “normal status” shown in Figure 15 . If a temporary increase in transferred data occurs and the data
transfer speed between the primary and secondary arrays is exceeded, excess data is temporarily
stored in journal volumes.
Continuous Access XP Journal user guide 63