HP Enterprise Cluster Master Toolkit User Guide (5900-2131, December 2011)

Figure 1 Oracle database storage hierarchy without and with ASM
Why ASM over LVM?
As mentioned above, we require ASM disk group members in Serviceguard configurations to be
raw logical volumes managed by LVM. We leverage existing HP-UX capabilities to provide
multipathing for LVM logical volumes, using either the PV Links feature, or separate products such
as HP StorageWorks Secure Path that provide multipathing for specific types of disk arrays. Another
reason for using LVM is that, it is the responsibility of Serviceguard to provide the necessary I/O
fencing when it provides failover for a single-instance Oracle database instance. Serviceguard
ensures I/O fencing for a failover package via a feature in the volume manager - exclusive activation
of volume groups. Other advantages of the "ASM-over-LVM" configuration are as follows:
• ASM-over-LVM ensures that the HP-UX devices used for disk group members will have the
same names (the names of logical volumes in LVM volume groups) on all nodes, easing ASM
configuration.
• ASM-over-LVM protects ASM data against inadvertent overwrites from nodes inside/outside
the cluster. If the ASM disk group members are raw disks, there is no protection currently
preventing these disks from being incorporated into LVM or VxVM volume/disk groups.
The disadvantages of the ASM-over-LVM configuration are as follows:
• Additional configuration and management tasks are imposed by the extra layer of volume
management (administration of volume groups, logical volumes, physical volumes).
• There is a small performance impact from the extra layer of volume management.
Configuring LVM Volume Groups for ASM Disk Groups
We require ASM disk group members in Serviceguard configurations to be raw logical volumes
managed by LVM. But these logical volumes presented to ASM should resemble raw disks, as far
as possible. Hence, each LVM logical volume (LV) used as a member of an ASM disk group is
required to be laid out to occupy the usable space, in contiguous fashion, of exactly one single
physical volume (PV). This implies that the LV:
• should be contiguous,
• should not be striped or mirrored,
Supporting Oracle ASM instance and Oracle database with ASM 29