Support of Oracle RAC ASM with SGeRAC, January 2008

HP Availability Clusters Solutions Lab 1/25/2008 Page 7
size of 16
Why ASM over SLVM?
As mentioned above, we require ASM disk group members in SGeRAC A.11.17 (or later)
configurations to be raw logical volumes managed by SLVM.
The main reason for this requirement is to expedite ASM support with SGeRAC. We leverage existing
HP-UX capabilities to provide multipathing for SLVM 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. Other advantages of the "ASM-over-SLVM" configuration are as follows:
• ASM-over-SLVM ensures that the HP-UX devices used for disk group members will have the
same names (the names of logical volumes in SLVM volume groups) on all nodes, easing
ASM configuration.
• ASM-over-SLVM 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-SLVM 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
• SLVM has some restrictions in the area of online reconfiguration, the impact of which will
be examined later in this document
Configuring SLVM Volume Groups for ASM Disk Groups
Ideally, ASM disk group members should be raw disks or array logical units, but due to the lack of
built-in multipathing we require them to be SLVM raw logical volumes in SGeRAC configurations. But
we would still like these logical volumes presented to ASM to resemble raw disks, as far as possible.
Hence, each SLVM 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,
• should not span multiple PVs
• and should not share a PV with other LVs.