Managing Serviceguard Extension for SAP on Linux (IA64 Integrity and x86_64), February 2008

Planning a File System Layout for SAP in a Serviceguard/LX Cluster Environment
About Impacted File Systems
Chapter 2 49
“LOCAL mount” or “SHARED mount”
The first step for each of the file systems used in an SAP configuration is
to classify if the mount point will be of type SHARED or of type LOCAL.
The disadvantage of a LOCAL mount point is that any changes to the
contents of these file systems will require a copy to all cluster nodes to
keep the contents of the file systems synchronized.
The above example diagram shows a two node / node cluster
configuration but up to 16 node / node configurations are supported with
Linux. So for A 16 node cluster 16 copies of the local file systems have to
be made requiring a considerable administration effort.
If possible therefore the preferred option would be to use type SHARED
for all mount points.
LOCAL mount There are several reasons a LOCAL mount point is
required in an SAP configuration:
• The first reason is that some files used in an SAP configuration are
not cluster aware. As an example consider the case of SAP file
/usr/sap/tmp/coll.put. This file contains system performance
data collected by the SAP performance collector. If the file system
/usr/sap/tmp is shared with other cluster nodes the SAP
performance collector running on those systems would write into the
same file system and file coll.put – causing this shared file to become
corrupted. Therefore this file system has to be mounted with a
LOCAL mount point.
• A second reason could be performance reasons. A locally mounted file
system will achieve higher I/O throughput than a NFS mounted file
system.
• A third reason could be the wish of having SAP binaries available
locally. In case of a network (and therefore NFS) failures the binaries
could still be executed. On the other side if there is a NFS failure
other SAP components will probably also fail.
As mentioned above, local copies of the file system contents require
administration overhead to keep them synchronized. Serviceguard
provides a tool cmcp(1m) that allows easy replication of a single file to all
cluster nodes.