Figure 1 Oracle database storage hierarchy without and with ASMWhy 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,

28 Using the Oracle Toolkit in an HP Serviceguard Cluster