User's Guide

Configuring PV Links
The examples in the previous sections describe the use of the vgimport and vgexport commands
with the -s option. In addition, the mk1VGs script uses a -s in the vgexport command, and the
mk2imports script uses a -s in the vgimport command. Optionally, remove this option from
both commands if using PV links. The -s option to the vgexport command saves the volume
group id (VGID) in the map file, but it does not preserve the order of PV links. To specify the exact
order of PV links, do not use the -s option with vgexport, and in the vgimport command,
enter the individual links in the desired order, as in the following example:
# vgimport -v -m mapfilename vgname linkname1 linkname2
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 with EMC
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
created using either EMC PowerPath or RDF Enginuity Consistency Assist (RDF-ECA) feature of
Solutions Enabler. In a consistency group, Symmetrix tracks the I/Os that are written to the devices.
If an I/O cannot be written to a remote Symmetrix because a remote device or an RDF link has
failed, the data flow to the other Symmetrix will be halted in less than one second. Once mirroring
is resumed, any updates to the data is propagated with normal SRDF operation.
Figure 46 shows when there is a break in the links between two of the Symmetrix frames, the use
of consistency groups (depicted as dashed oval lines) ensures that the other two links are also
suspended.
Figure 46 2 X 2 Node and Data Center Configuration with Consistency Groups
Setting up M by N Configurations
Metropolitan clusters using EMC SRDF can be built in configurations that use more than two EMC
Symmetrix disk arrays. In such configurations, M arrays located in Data Center A may be connected
to N arrays located in Data Center B. This section describes how to set up an M by N configuration
using EMC Solutions Enabler and HP-UX commands. It is assumed that either Symmetrix PowerPath
software is installed or RDF-ECA feature of Solutions Enabler is enabled on all nodes and the
Building a Metrocluster Solution with EMC SRDF 265