HP XP Continuous Access Software - Manual (Page 61)

The following factors affect the speed of writing to and reading from journal volumes, particularly when a
temporary communications path failure occurs between the primary and the secondary arrays or when
the amount of data transferred from hosts to the primary array increases:
• RAID configuration of the RAID groups that will contain journal volumes
• Types of physical volumes in the RAID groups that will contain journal volumes
• Frequency of access to non-journal volumes in the RAID groups that will contain journal volumes
• Data transfer speed required for the non-journal volumes
• Disk usage rate for RAID groups
The journal volumes' capacity affects the time during which data transfer with hosts can continue without
being influenced by a temporary communications path failure between the primary and secondary arrays
or an increase in the data to be transferred from hosts to the primary array.
Planning process
Data transfer speed required for journal volumes
Figure 13 illustrates how the data transfer speed (that is, the amount of data to be transferred per unit of
time) changes as time elapses, showing different types of data transfer speeds with Continuous Access
XP Journal. Data transfer speed between hosts and the primary array goes through two phases. In
one phase, the data transfer speed remains almost unchanged. In the other phase, the data transfer
speed temporarily increases.
Figure 13 Data transfer speed of journal volumes
As illustrated in Figure 13 , the data transfer speed (that is, the speed for reading and writing) of journal
volumes in the master journal group must exceed the amount of temporary increase in data to be
transferred. If the journal volumes' data transfer speed is below the amount of temporary increase in
data to be transferred, journal data for the temporary increase in data to be transferred is not stored in
journal volumes in a timely manner.
In
Figure 13 , the data transfer speed between the primary and the secondary arrays indicates the
transfer speed of journal data between the primary and secondary arrays. For more information, see
Journal volumes in restore journal groups .
RAID group and journal group configurations
A RAID group can consist of physical volumes with a different number of revolutions, physical volumes with
different capacities, and physical volumes with different RAID configurations (for example, RAID-1 and
RAID-5). A RAID group's data transfer speed is affected by the physical volumes and RAID configurations.
Continuous Access XP Journal user guide 61