HP XP Continuous Access Software - Manual (Page 55)

A journal volume's data transfer speed depends on the data transfer speed of the RAID group to
which the journal volume belongs. One RAID group can consist of one or more logical volumes,
including journal volumes. Therefore, if frequent accesses are made to non-journal volumes in a
RAID group, relatively fewer accesses can be made to journal volumes in the same RAID group,
which can cause a drop in the journal volumes' data transfer speed. To avoid this drop, you should
consider, for example, relocating journal volumes and frequently accessed non-journal volumes
(that is, placing journal groups in one RAID group and placing frequently accessed non-journal
volumes in another RAID group).
Arranging journal volumes
The following indicates the data transfer speed of journal volumes mentioned in Data transfer speed
required for journal volumes .
If a host attempts to write data of aa kilobytes and the number of write I/Os per second is bb, the
data transfer speed required for journal volumes is calculated as follows:
Data transfer speed of journal volumes > aa x bb ÷ 1,024 (MB/sec)
The journal volumes' data transfer speed must exceed the data transfer speed from hosts to the
primary array. You must consider the data transfer speed (that is, the speed for writing and reading)
required for journal volumes, determine the physical volume and RAID configurations of RAID
groups, and create journal volumes in the RAID groups.
Journal volume capacity
In Figure 14 (page 55) , the size of the shaded area indicates the amount of journal data to be
stored in journal volumes as a result of a temporary increase in data transferred.
If a temporary communications path failure occurs between the primary and secondary arrays,
journal transfers between the primary and secondary arrays temporarily stops. If hosts transfer
data to the primary array while journal transfers between the primary and secondary array are
temporarily stopped, all data transferred from the hosts to the primary array is stored as journal
data in the journal volumes.
Figure 14 Influence of data transfer speed on journal volume capacity
The following factors determine the required journal volume capacity:
•
Period of time during which data transfers can continue between hosts and the primary local
array when a temporary increase in transferred data occurs or when a communications path
failure occurs between the primary and secondary local arrays
•
Data transfer speed for the period of time mentioned previously
To determine the journal volume capacity, use the following formula:
Planning for journal volumes 55