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 380
If the network is configured to prevent the application from
communicating with its clients under these circumstances, the
clients will not receive any acknowledgement of these writes and
after the failover will re-transmit them, and the writes will be
committed and acknowledged at DC2. This is the desired outcome;
HP recommends you configure the network such that when links
between the sites fail, the communication links to the application
clients are also shut down.
In the case of an XDC configuration such as that shown in Figure
1-4, there is an additional variable in the possible failure scenarios.
Instead of a DWDM link, in this configuration there are two separate
LAN and FC links which can experience failure independent of each
other. If the network links between the sites fail within a very short
period (on the order of 1 second) after t1 (after the storage links had
failed), the XDC software on DC1 will not have time to inform the
XDC on DC2 of the failure. So DC2 assumes that there were no
updates after t1, but there may have been.
When this scenario occurs, disk writes continue on DC1 until t3. In
this case, the effective value of the RPO_TARGET parameter is greater
than the expected value of 0.
Again, if the network is set up in such a way that when the links
between the sites fail, the communication links to the application
clients are also shut down, then the unintended writes are not
acknowledged and have no long term effect.
IMPORTANT The value you set for RPO_TARGET must be more than the value you
set for the RAID_MONITOR_INTERVAL parameter. By default, the
RAID_MONITOR_INTERVAL parameter is set to 30 seconds.
For example: RPO_TARGET=60 seconds
• MULTIPLE_DEVICES AND COMPONENT_DEVICES
Parameter RAID_DEVICE [ ] specifies the MD devices that are used
by a package. You must begin with RAID_DEVICE[0], and increment
the list in sequence. Component device parameters DEVICE_0[ ] and
DEVICE_1[ ] specify the component devices for the MD device of the
same index.
For example, if a package uses multiple devices such as