HP XP Continuous Access Software - Manual (Page 56)

Journal volume capacity > V
H-M
- V
M-R
x t
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 (page 56) . 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.
Figure 15 Data transfer speed between primary and secondary arrays
56 XP Continuous Access Journal operations requirements and restrictions