Preparing a Cluster for MetroCluster/SRDF

Installing the Necessary Software

Before any configuration can begin, you need to make sure the following software is installed on all nodes:

Setting up Hardware for 1 by 1 Configurations

Ensure that the Symmetrix disk arrays are correctly cabled using PV links to each node in the cluster that will run packages that access data on the Symmetrix.

See the SymCLI manual for instructions on creating the appropriate pseudo device files.

Setting up Hardware for M by N Configurations

You can configure up to four Symmetrix disk arrays in the following combinations:

	WARNING! M by N configurations cannot be used with R1/R2 swapping.

Figure 4-9 “2 by 1 Node and Data Center Configuration” shows a 2 by 1 configuration with BCVs.

Figure 4-9 2 by 1 Node and Data Center Configuration

The figure indicates R1 volumes at Data Center A and R2 volumes and BCVs at Data Center B for pkg A and pkg B.

Figure 4-10 “2 by 2 Node and Data Center Configuration” shows a 2 by 2 configuration with R1 volumes for pkg A and pkg B on the Symmetrix frames located in Data Center A and R2 volumes and BCVs at Data Center B. Many of the examples given later in this chapter are based on this configuration.

Figure 4-10 2 by 2 Node and Data Center Configuration

Figure 4-11 “Bidirectional 2 by 2 Configuration” below shows a bidirectional 2 by 2 configuration with additional packages on node3 and node4, and R1 and R2 volumes at both data centers. In this configuration, R1 volumes for pkg A and pkg B are at Data Center A, and R2 volumes are at Data Center B. R1 volumes for pkg C and pkg D are at Data Center B, and R2 volumes are at Data Center A.

Figure 4-11 Bidirectional 2 by 2 Configuration

Grouping the Symmetrix Devices at Each Data Center

The use of R1/R2 devices in M by N configurations of multiple Symmetrix frames is enabled by means of consistency groups. A consistency group is a set of Symmetrix RDF devices that are configured to act in unison to maintain the integrity of a database. Because MetroCluster/SRDF works at the device group level, the consistency group is implemented and managed as a single device group even though it spans multiple Symmetrix frames. Consistency groups are managed by the EMC PowerPath product. When PowerPath is installed, the Symmetrix tracks the I/Os that are written to the devices in the consistency group. If an I/O cannot be written to a remote Symmetrix due to a failure of a remote device or an RDF link, the data flow to the other Symmetrix will be halted in less than one second. Once mirroring is resumed, any updates will propagate with normal SRDF operation.

Figure 4-12 “2 X 2 Node and Data Center Configuration with Consistency Groups” shows that when there is a break in the links between two of the Symmetrix frames, the use of consistency groups (dashed oval lines) ensures that the other two links are also suspended.

Figure 4-12 2 X 2 Node and Data Center Configuration with Consistency Groups

Building the Symmetrix CLI Database

The Symmetrix CLI (Command Line Interface) should be installed on all nodes running packages that use data on the EMC Symmetrix disk arrays. Create the SymCLI database on each system using the following steps. For complete information, refer to the Symmetrix SymCLI manual.

Issue the following command on each node after the hardware is installed:

# symcfg discover

This builds the CLI database on the node.

You can display what is in the SymCLI database with the commands:

If you have not configured the SymCLI database, you will see an error:

The Symmetrix configuration could not be loaded for a locally attached Symmetrix

	NOTE: Make sure that you do not set the SYMCLI_SID and SYMCLI_DG environment variables before running the symcfg command. These environment variables limit the amount of information gathered when the SymCLI database is created, and will not give you a complete database. Also, the SYMCLI_OFFLINE variable should not be set. This environment variable disables the command line interface.

Determining Symmetrix Device Names on Each Node

To correctly specify the device file names when creating Symmetrix device groups, you need to know how the HP-UX device files map to the R1 and R2 Symmetrix devices. Use the following steps to gather the necessary information.

	NOTE: The Symmetrix device number may be the same or different in each of the Symmetrix units for the same logical device. In other words, the device number for the logical device on the R1 side of the SRDF link may be different from the device number for the logical device on the R2 side of the SRDF link. The Symmetrix logical device numbers in these examples were configured to be the same number so that the cluster is easier to manage. If you are reconfiguring an existing cluster, the Dev and RDev devices will probably not be the same number. When you are determining the configuration for the Symmetrix devices for a new installation, it is recommended that you try to use the same Symmetrix device number for the R1 devices and the R2 devices. It is also recommended that the same target and LUN number be configured for all nodes that have access to the same Symmetrix logical device.