HP XP7 Storage - Manual (Page 22)

HP recommends a capacity of 6 GB or more. Journal volumes cannot be registered if capacity is
lower than 1.5 GB.
To calculate journal size
1.
Follow the instructions for “Measuring write-workload” (page 20) .
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 two-hour 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 33) .
NOTE: If you are planning for disaster recovery, the secondary system must be large enough to
handle the production workload, and therefore, must be the same size as master journals.
Planning journals
Cnt Ac-J manages pair operations for data consistency through the use of journals. Cnt Ac-J 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 ensure 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.
The following information about journal volumes and journals will help you plan your journals.
•
A journal consists of one or more journal volumes and associated data volumes.
•
A journal can have only P-VOLs/master journals, or S-VOLs/restore journals.
•
A journal cannot belong to more than one storage system (primary or secondary).
•
All the P-VOLs, or S-VOLs, in a journal must belong to the same storage system.
•
Data volumes in different virtual storage machines cannot be registered in the same journal.
•
Master and restore journal IDs that are paired can be different.
If using a consistency group number, the consistency group number of the P-VOL and S-VOL
must be the same.
•
Each pair relationship in a journal is called a "mirror". Each pair is assigned a mirror ID. The
maximum number of mirror IDs is 4 (0 to 3) per system.
•
When Cnt Ac-J and Cnt Ac-J Z are used in the same system, individual journals must be
dedicated either to one or the other, not both.
22 Planning volumes, XP7 systems