Planning The Data Path; Data Path Design Workflow; Sizing Bandwidth; Five Sizing Strategies - HP XP P9500 User Manual

4 Planning the data path

A data path must be designed to manage your organization's throughput to the remote site.
Appropriate bandwidth, required number of ports, and the Fiber Channel data path configuration
you use help insure that your update data arrives at the remote site in a time consistent with your
organization's RPO.
This chapter provides instructions for calculating bandwidth and designing the data path network.

Data path design workflow

To set up a data path, you need to establish the following:
The amount of bandwidth necessary to move the data generated by your host application
under all I/O conditions
Ports that can either send or receive data
Understand Fibre Channel types and number of switches required
A data path
This chapter discusses these topics in detail.

Sizing bandwidth

You purchase bandwidth according to the amount of data that will be transferred from the primary
to the secondary system within a certain amount of time.
If the data path network cannot keep pace with the flow of data, the data is saved in the journal
until additional bandwidth capacity becomes available. If the journal also cannot keep up, the
pair relationships will be lost and a resync or possibly a new initial copy must be performed.
In general, bandwidth is expensive. Adding capacity to a journal volume is relatively inexpensive.
But the more data that accumulates in the journal, the further the secondary image lags behind the
production volumes. Therefore, sizing bandwidth is a trade-off between expense and keeping your
secondary volumes as close in time as you require with the primary volumes.

Five sizing strategies

The following sizing strategies are provided to help you work out an approach to sizing bandwidth.
Be aware that these are not the only strategies you can use.
Size bandwidth to peak workload. This results in the smallest time difference between data in
the P-VOL and S-VOL. Identify peak workload on the production disks, then add extra capacity
to accommodate packet loss and protocol overhead. RPO is as near zero as possible when
bandwidth is sized to peak workload.
Size bandwidth to peak workload rolling average. The rolling average is less than peak but
more than average. This guarantees that at some point data will accumulate in the journal,
but most of the time it will not. The RPO in this case is based on the size of the journal when
the data path capacity is exceeded and the RPO will vary over time depending on the specific
workload being presented at any point in time. You must determine that the maximum RPO
provided is within the limit of your requirements.
Size bandwidth to typical workload. When bandwidth is sized to typical write-workload, and
an extended peak workload is experienced, excess write-data (the data greater than the data
path capacity) is written to journal. This excess data is delayed for subsequent FIFO transmission
to the remote site when network capacity becomes available. The amount of data written to
the journal and accumulated there is proportional to the amplitude and duration of the workload
surge.
Data path design workflow 35