HP StorageWorks XP Remote Web Console Software - Manual (Page 69)

Journal Volume Capacity
In Figure 14 on page 69, the size of the shaded area indicates the amount of journal data to be stored in
journal volumes as a result of temporary increase in data transferred.
If a temporary communication path failure occurs between the primary storage system and the secondary
storage system, journal transfers between the primary storage system and the secondary storage system
will stop temporarily. If hosts transfer data to the primary storage system while journal transfers between
the primary storage system and the secondary storage system are stopped temporarily, all the data
transferred from hosts to the primary storage system will be stored as journal data in journal volumes.
Data transfer
Data transfer speed
between hosts and
the MCU
Data transfer speed of journal volumes
Data transfer speed
between the MCU and the RCU
Time
speed
Normal status Temporary increase in
data to be transferred
Figure 14 Data Transfer Speed with URz (Influence on Journal Volume Capacity)
The following factors determine the required journal volume capacity:
• the period of time during which data transfer can continue between hosts and the primary storage
system when a temporary increase in transferred data occurs or when a communication 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, VH-M is the data
transfer speed between hosts and the primary storage system. VM-R is the data transfer speed between
the primary storage system and the secondary storage system. t is the length of time during which
data transfer can continue.
Journal volume capacity > VH-M - VM-R × t
If you want to calculate journal volume capacity that is required when a communication path failure occurs
between the primary storage system and the secondary storage system, please assign 0 (zero) to VM-R.
The total capacity of journal volumes in each journal group must exceed the value illustrated above.
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 (i.e., change the copy direction). If you want 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 these data volumes. Therefore, when you do planning
on master journal volumes and restore journal volumes, you need to apply the same scheme to both
master and restore journal volumes.
Hitachi Universal Replicator for z/OS user guide for XP12000/XP10000 Disk Arrays and SVS 200 69