HP StorageWorks XP Remote Web Console Software - Manual (Page 111)

Figure 51 Data Transfer Speed with XP Continuous Access Journal (Influence on Journal Volume
Capacity)
The following factors determine the required journal volume capacity:
•
The period of time during which the data transfer can continue between the hosts and the
primary storage system when a temporary increase in transferred data occurs or when a
communications path failure occurs between the primary and secondary storage system
•
The data transfer speed for the period of time that is mentioned above
To determine the journal volume capacity, use the following formula. In this formula, V
H-M
is the
data transfer speed between the hosts and the primary storage system. V
M-R
is the data transfer
speed between the primary storage system and the secondary storage system. t is the length of
time during which the data transfer can continue.
Formula 1:
Journal volume capacity > V
H-M
- V
M-R
× t
To calculate the journal volume capacity that is required when a communications path failure
occurs between the primary storage system and the secondary storage system, assign 0 (zero) to
V
M-R
.
The total capacity of the journal volumes in each journal group must exceed the value illustrated
in Formula 1above.
When the data in the primary data volume of the XP Continuous Access Journal delta resync pair
is updated, if the journal data that exceeds the 70 percent of the journal volume capacity on the
primary site of the delta resync pair is generated, the delta resync operation will fail. Therefore,
adopt the larger value for the journal volume capacity on the primary site of the XP Continuous
Access Journal delta resync pair by comparing the value of the Formula 1 and Formula 2.
In Formula 2, V is the data transfer speed between a host and the primary storage system. t is the
length of time until the delta resync operation starts.
Formula 2:
Journal volume capacity > (V x t x 1.5)
Journal Volumes in Restore Journal Groups
When a failure occurs at a host, one of the failure recovery measures is to reverse the primary
data volume and the secondary data volume (that is, change the copy direction). To reverse the
primary data volume and the secondary data volume, usually you must ensure that the data transfer
speed is the same before and after you reverse the data volumes. Therefore, when you do planning
on master journal volumes and restore journal volumes, you need to apply the same scheme to
both the master and restore journal volumes.
Planning of Journal Volumes 111