Serviceguard Extended Distance Cluster (EDC) with VxVM/CVM Mirroring on HP-UX, May 2008

Disk Group configuration database on disk
VxVM creates and maintains configuration information in the private region of several disks
belonging to a disk group. Multiple disks in different enclosures are automatically selected to store
persistent copies of the VxVM object information.
There is normally no need to manually change the choices that VxVM makes. To verify that disks at
both sites are configured to hold configuration information, use the vxdg
command described in
figure 5.
Figure 5. To verify the placement of the configuration information, use the following command:
$ vxdg list dgEDC | grep config | grep online
config disk DC1-XP12k1_0 copy 1 len=640 state=clean online
config disk DC1-XP12k1_5 copy 1 len=640 state=clean online
config disk DC1-XP12k1_7 copy 1 len=640 state=clean online
config disk DC2-EVA8k1_2 copy 1 len=640 state=clean online
config disk DC2-EVA8k1_7 copy 1 len=640 state=clean online
In addition to verifying which disks contain configuration information, the vxdg command can be
used to configure additional disks to hold copies of the configuration information increasing
redundancy. The vxdg(1M) man page explains how to do this.
Mirror (plex) configuration
Each volume must have a complete copy of the data at each data center. This is achieved by volume
manager mirroring functionality. In VxVM/CVM terminology, a mirror is referred to as a “plex”. In an
EDC, each volume consists of 2 plexes—one in each data center. Creating more than one plex per
site on a highly available storage system does not provide sufficient value to justify the cost of the
storage and the increased performance requirements for write operations. Typically the individual
array has sufficient redundancy to guard against disk failures. The mirror functionality of a volume
manager is used in an EDC to guard against the failure of an entire storage system.
If a volume is created with the vxassist command, the following directives are used to create a
two-way mirrored volume:
• layout=mirror
Configures the volume as mirrored. If a plex is supposed to be turned into a break-off instant
snapshot volume for maintenance reasons, it must be a non-layered volume with either a mirror or
mirror-stripe layout.
• nmirror=2
Configures the number of mirrors (plexes)—one for each of the two data centers in the EDC.
Since EBN schema is used for naming the disks, the following directives cause VxVM to mirror across
enclosures and place one plex per volume on each enclosure:
• mirror=enclr
This option directs vxassist to mirror across enclosures. Together with the following two options:
– enclr:DC1-XP12k1
– enclr:DC2-EVA8k1
It creates one plex on a disk/LUN of enclosure DC1-XP12k1 and one plex on a disk/LUN of
enclosure DC2-EVA8k1.
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