Managing Serviceguard Extension for SAP on Linux (IA64 Integrity and x86_64), April 2009

About Storage Options
For each of the above listed file system scenarios the following questions need to be answered:
1. Whether it needs to be kept as a LOCAL copy on internal disks of each node of the cluster. The file
system requires a LOCAL mount point.
2. Which of the file systems have to be SHARED by all cluster nodes on a SAN storage, but have to be
mounted in (SHARED EXCLUSIVE) mode by the cluster node that the SAP instance failed over to and
will run that instance.
3. Which of the file systems can be SHARED in (SHARED NFS) mode in the cluster by all cluster nodes
(e.g. access to the file system is required from more than one cluster node at the same time)?
Figure 2-2 Two node cluster with LOCAL and SHARED mounted / connected storage
LOCAL Storage
The LOCAL connected storage device "Storage 1" can only be mounted by cluster node "Node 1", the
storage device "Storage 4" can only be mounted by cluster node "Node 2". So "Storage 1" and "Storage
2" access is LOCAL for these cluster nodes. Sharing of data residing on “Storage 1” or “Storage 2” with
the other cluster nodes is not required and the other cluster nodes do not require any direct access to this
storage device.
SHARED Storage
The SHARED connected storage "Storage 2” and Storage 3” can be mounted from any of the cluster nodes
“Node 1” or “Node 2”. Storage will be SHARED between the cluster nodes. At some point and time one
or more of the cluster nodes will require access to the shared storage. Depending on the type of "sharing"
this can further be classified as SHARED NFS or SHARED EXCLUSIVE.
In the SHARED EXCLUSIVE case the storage device is accessible from any of the cluster nodes but as some
point and time only one cluster node will mount the storage exclusively in the cluster. It will be that cluster
node that requires that file system to run the SAP instance.
In the SHARED NFS case the file systems are mounted via NFS and available to all cluster nodes (via NFS
export / import) at all times.
"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 / cluster configuration. 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.
32 Planning a File System Layout for SAP in a Serviceguard/LX Cluster Environment