HP XP P9500 Storage - Manual (Page 25)

where:
•
V is the data transfer speed between a host and the primary storage system.
•
t is the length of time until the delta resync operation starts.
CAUTION: The journal volume capacity is recommended to be over 6 GB. If the capacity is
below 6 GB, the system performance is unassured because the following problems may occur.
•
The new data cannot be stored because the journal volume is full and host performance is
reduced.
•
The performance of the initial copy goes down because the journal volume is full.
•
The journal group is suspended because the journal volume is full.
•
The Usage Monitor value is invalid.
Calculating the journal size
You calculate the size your journal volumes using the write-workload and RPO.
To calculate journal size
1.
Follow the instructions for “Measuring write-workload” (page 23) .
2. Use your system’s peak write-workload and your organization’s RPO to calculate journal size.
For example:
RPO = 2 hours
Write-workload = 30 MB/sec
Calculate write-workload for the RPO. In the example, write-workload over a 2 hours period
is calculated as follows:
30 MB/second x 60 seconds = 1800 MB/minute
1800 MB/minute x 60 minutes = 108,000 MB/hour
108000 MB/hour x 2 = 416,000 MB/2 hours
Basic journal volume size = 416,000 MB (416 GB)
Journal volume capacity and bandwidth size work together. Also, your strategy for protecting your
data may allow you to adjust bandwidth or the size of your journal volumes. For a discussion on
sizing strategies, see
“Five sizing strategies ” (page 39) .
NOTE: If you are planning for disaster recovery, the remote array must be large enough to handle
the production workload, and therefore, must be the same size as master journals. If not planning
a disaster recovery solution, remote journal volumes may be smaller than master journal volumes.
Planning journals
Continuous Access Journal Z manages pair operations for data consistency through the use of
journals. Continuous Access Journal Z journals enable update sequence consistency to be maintained
across a group of volumes.
Understanding the consistency requirements for an application (or group of applications) and their
volumes will indicate how to structure journals.
For example, databases are typically implemented in two sections. The bulk of the data is resident
in a central data store, while incoming transactions are written to logs that are subsequently applied
to the data store.
If the log volume “gets ahead” of the data store, it is possible that transactions could be lost at
recovery time. Therefore, to insure a valid recovery image on a replication volume, it is important
that both the data store and logs are I/O consistent by placing them in the same journal.
Sizing journal volumes 25