HP Serviceguard Extended Distance Cluster for Linux A.01.01 Deployment Guide, Third Edition, May 2008

Configuring your Environment for Software RAID
Configuring the Legacy Package Control Script and RAID Configuration File
Chapter 378
Because some timers are affected by polling, the value of this
parameter can vary by approximately 2 seconds.
This also requires that the minimum value of this parameter is 2
seconds if a small value is necessary. Change the value of
RPO_TARGET, if necessary, after considering the cases discussed
below.
Cases to Consider when Setting RPO_TARGET
RPO_TARGET allows for certain failure conditions when data is not
synchronized between the two sites.
For example, let us assume that the data storage links in Figure 1-4
fail before the heartbeat links fail. In this case, after the time
specified by Link_Down_Timeout has elapsed, a package in
Datacenter1 (DC1) will continue updating the local storage, but not
the mirrored data in datacenter2 (DC2). While the communication
links must be designed to prevent this situation as far as possible,
this scenario could occur and may last for a while before one of the
sites fails.
NOTE For more information on how access to disks is disabled in certain
failure scenarios, see “Setting the Value of the Link Down Timeout
Parameter” on page 67.
Let us consider a few failure scenarios and the impact of different
RPO_TARGET values. The discussion below is based on the timeline
and events shown in Figure 3-1.