HP StorageWorks XP Remote Web Console Software - Manual (Page 50)

50 Continuous Access XP Journal
relatively fewer accesses can be made to journal volumes in the same RAID group, which can cause a
drop in the data transfer speed of the journal volumes. To avoid this drop, you should consider, for
example, relocating the journal volumes and the frequently accessed non-journal volumes (that is, placing
the journal groups in one RAID group and placing the frequently accessed non-journal volumes in another
RAID group).
Arranging 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 data transfer speed of journal volumes must exceed the data transfer speed from hosts to the primary
disk array. You must consider the data transfer speed (that is, the speed for writing and reading) required
for journal volumes, determine the physical volume configuration and RAID configuration of RAID groups,
and create journal volumes in the RAID groups.
Journal volume capacity
In Figure 12 , 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 disk arrays,
journal transfers between the primary and secondary disk arrays stops temporarily. If hosts transfer data to
the primary disk array while journal transfers between the primary and secondary disk array are stopped
temporarily, all data transferred from the hosts to the primary disk array is stored as journal data in journal
volumes.
Figure 12 Influence of data transfer speed on journal volume capacity
The following factors determine required journal volume capacity:
• Period of time during which data transfer can continue between hosts and primary local disk arrays
when a temporary increase in transferred data occurs or when a communication path failure occurs
between the primary and secondary local disk arrays
• Data transfer speed for the period of time mentioned previously
To determine the journal volume capacity, use the following formula:
Journal volume capacity > V
H-M
- V
M-R
x t
where:
• V
H-M
is the data transfer speed between the hosts and primary disk array
• V
M-R
is the data transfer speed between the primary and secondary disk arrays
• t is the length of time during which data transfer can continue
To calculate the journal volume capacity required when a communications path failure occurs between the
primary and secondary disk arrays, assign 0 (zero) to V
M-R
.