Staring in Oracle 11g Real Application Clusters (RAC), we see a new process called
Oracle High Availability Services or OHAS for short.
OHAS is implemented via a new daemon process which is
called ohasd on the OS layer, and the 11g documentation notes that
this Oracle High Availability Services Daemon (OHASD) anchors the lower part
of the Oracle Clusterware stack, which consists of processes that facilitate
cluster operations in RAC databases. This includes the GPNPD, GIPC,
MDNS and GNS background processes.
It appears that RAC DBA's will continue to use the
traditional crsctl commands for cluster management and that there
are no direct commands for Oracle High Availability Services.
The 11g docs note the jobs of the OHAS Oracle High
Availability Services Stack:
The list in this section describes the processes that
comprise the Oracle High Availability Services stack.
Grid Plug and Play
(GPNPD): GPNPD provides access to the Grid Plug and Play profile,
and coordinates updates to the profile among the nodes of the cluster to
ensure that all of the nodes node have the most recent profile.
Interprocess Communication (GIPC): A helper daemon for the
communications infrastructure. Currently has no functionality; to be
activated in a later release.
Name Service (mDNS): Allows DNS requests. The mDNS process is a
background process on Linux and UNIX, and a service on Windows.
Naming Service (GNS): A gateway between the cluster mDNS and
external DNS servers. The
gnsd process performs name
resolution within the cluster.
Enabling Oracle High Availability Services (OHAS) in
11gr2 and beyond
To enable OHAS for each RAC node, you issue the
crsctl enable crs command, and this will cause OHAS to autostart when
each node re-boots. To verify that OHAS is running, check for the
CRS-4123 message in your alert log:
CRS-4123: Oracle High Availability
Services has been started.
ohasd is starting
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