HP Serviceguard Extended Distance Cluster for Linux A.11.20.10 Deployment Guide, December 2012

3.3 Configuring the Storage Environment
After setting up the hardware as described in the Extended Distance Cluster Architecture section
and installing the Extended Distance Cluster software, see the following sections that describe each
of these processes in detail.
3.3.1 Configuring the Storage
This section describes the configuration changes recommended for both single path and multiple
paths to storage as described in Figure 3 (page 15).
3.3.1.1 Setting the Value of the Link Down Timeout Parameter
After you install, you must set the Link Down Timeout parameter for the Fibre Channel cards
to a duration equal to the cluster reformation time. The value of cluster reformation time parameter
depends on the heartbeat interval and the node timeout values configured in a particular cluster.
To get the cluster reformation time set for a particular cluster:
cmviewcl –vf line | grep max_reformation_duration
This parameter prevents any data being written to a disk when a failure occurs. The value of this
parameter must be set such that the disks are inaccessible for a time period which is greater than
the cluster reformation time. This parameter is important in scenarios where an entire site is in the
process of going down. By blocking further writes to the MD device, the two disks of the MD device
remain current and synchronized. As a result, when the package fails over, it starts with a disk
that has current data.
The Fibre Channel cards are configured to hold up any disk access and essentially hang for a
time period which is greater than the cluster reformation time when access to a disk is lost. This is
achieved by altering the Link Down Timeout value for each port of the card. Setting a value
for the Link Down Timeout parameter for a Fibre Channel card ensures that the MD device
hangs when access to a mirror is lost. However, the MD device resumes activity when the specified
hang period expires. This ensures that no data is lost.
This parameter is required to address a scenario where an entire datacenter fails but all its
components do not fail at the same time but undergo a rolling failure. In this case, if the access to
one disk is lost, the MD layer hangs and data is no longer written to it. Within the hang period,
the node goes down and a cluster reformation takes place. When the package fails over to another
node, it starts with a disk that has current data.
3.3.1.2 Configuring Single Path to Storage
For Fibre Channel cards, you can configure the timeout parameter using the Link Down Timeout
parameter. Link Down Timeout parameter, in turn can be configured using SANSurfer CLI tool
on Red Hat Enterprise Linux 5 and 6 or SUSE Linux Enterprise Server 11 SP1 and using
QConvergeConsoleCLI tool on SUSE Linux Enterprise Server SP2.
For Emulex cards, you can configure the timeout parameter using the Link Down Timeout
parameter that can be configured using HBAAnywhere tool.
3.3.1.3 Configuring Multiple Paths to Storage
In addition to configuring the Link Down Timeout parameter for each card as described in the
previous section, you need to configure the dev_loss_tmo parameter in the /etc/
multipath.conf file to a value greater than the cluster reformation time.
3.3.2 Using Persistent Device Names
Once the single path is configured, you need to create persistent device names using udev. In
cases of disk or link failure and subsequent reboot, it is possible that device names are renamed
or reoriented. Since the MD mirror device starts with the names of the component devices, a change
22 Configuring your Environment for Software RAID