HP StorageWorks XP24000 Continuous Access Journal Software User and Reference Guide, v01 (T5278-96001, June 2007)

reading) require d for the journal volumes, determine the physical volume configuration and RAID
configuration of the RAID groups, and create the journal volumes in the RAID groups.
Journal Volume Capacity
In Figure 33 on page 72, the size of the shaded a rea indicates the amount of journal data to be stored in
the journal volumes as a result of a temporary increase in the data transferred.
If a temporary communications 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 the hosts transfer data to the primary storage system while
journal transfers bet ween the primary storage system and the secondary storage system are stopped
temporarily, all the data transferred from the hosts to the primary storage system will be stored as journal
data in the j
ournal 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 33 D ata Transfer Speed with Continuous A ccess Journal (Infl uence on Journal
Volume Capac it y)
The following factors determine the required journal volume capacity:
• the period of time during which the data transfer can continue bet ween the hosts and the primary
storage system when a temporary increase in transferred data occurs or when a communications
path failure occurs between the primar y and secondary storage system.
• the data transfer speed for the p eriod of time that is mentioned above
To determine the journal volume capacity, use the following formula. In this formula, V
H-M
is the d ata
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.
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 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 (that is, change the copy direction). To reverse the primary
72
Preparing for Continuous Access Journal Operations