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Multiple Access Paths to Host

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

Multipathing generally allows two or more data paths to be used simultaneously for read/write operations. This enhances performance by automatically and equally dispersing data access across all the available paths. Many storage and storage software vendors offer this kind of reliability.

Some of the widely used solutions such as EMC?s Power Path, HP?s Secure Path, and Veritas?s Dynamic Multipathing solution will be examined next.

Power Path (Dell / EMC)

EMC?s Power Path helps to improve the high availability of the data or I/O path and manages the heavy storage load. It automatically detects host failures and recovers storage. Power Path is host-based software that runs on UNIX, Windows, and Linux servers. It can manage the protocols in both SCSI and fiber physical interconnects. Power Path creates a virtual power device that provides failure resistance and a load balanced path. Power devices are virtual objects that contain file systems or raw partitions. They are managed by a volume manager or host system. Each power device represents a LUN at the host level but remains available through multiple I/O paths. The detailed path is shown in Figure 5.11.

Figure 5.11: Power Path as implemented in EMC Clariion storage array

Secure Path

HP's Secure Path is multipath, high availability software that manages and maintains continuous data access to HP storage systems. There is no single point of failure from the server to the storage. Secure Path is host resident software that monitors the data paths between the server and the storage to increase the availability of the information. In the event a path failure is detected, Secure Path fails over the data path to an alternative path. When the original path becomes available, Secure Path can automatically fall back to the original path. Secure Path can also balance the workload among available paths to optimize system performance.

Dynamic MultiPathing

Dynamic Multipathing (DMP) is a feature of the Veritas Volume Manager. It offers greater reliability by providing a path failover mechanism. In the event a connection to a disk is lost, the system continues to access the critical data over the remaining sound connections to the disk. DMP also provides greater I/O throughput by balancing the I/O load uniformly across multiple I/O paths to the disk device.


PolyServe Matrix Server supports multipath I/O in its driver stack. MxMPIO offers solid redundancy in the event of an HBA failure. Additionally, MxMPIO offers I/O load balancing by multiplexing I/O for varying LUNs between the different HBAs in the system. Bringing this feature to the Intel-based server-clustering arena makes commodity-based datacenter solutions one step closer to reality.

Volume Management

The database is constructed and maintained on file system files or raw partitions, which are entities at the operating system level. They are directly attached to storage devices, NAS devices, or SAN volumes. There are several different layers of abstraction between the physical disks and the objects that the database uses. The physical drive is at the lowest level, next comes the LUNS (logical unit numbers) of manageable sizes employing an appropriate RAID level. LUNS are presented to the host system as physical objects.

The Logical Volume Manager (LVM) is a software module running at the host level and manages the physical objects and presents them as logical units that the application can use. The volume manager hides the physical attributes of the disks or LUNs by introducing virtualization. The volume manager can also execute the RAID operations if needed. Within the volume manager, storage devices are grouped into disk groups (e.g. Veritas disk group).

Within a disk group, a logical volume can be created with an optional or desired RAID level. Finally, the logical volume is either presented as a file system after mounting, or presented as a raw partition for application use. The VM tool provides a very flexible way of managing the volumes and file systems. Without a volume manager, LUNS are physical disks with partitions.

Figure 5.12: Volume Configuration using Veritas Volume Manager

Thus the volume manager hides the details about where data is stored in the hardware from the entire system. Volume management allows the editing of the storage configuration, for example, setting up a software RAID or extending volume size, etc., without actually changing anything on the hardware side. Some volume manager tools allow snapshot copies to be taken of volumes to move or backup. Thus, for either creating the raw partitions (or volumes) or setting up of file systems, the volume manager is a handy tool.

Figure 5.12 above shows how the LVM provides volumes and file systems to a host. LVM hides all the complexity in the backend of the disk drives or the storage units. The database system will just interact with the volumes presented by the LVM.

The Veritas volume manager has been in use on many platforms like Solaris, HP, and AIX. Now, Veritas even has the Volume Manager to support Windows systems. On the Linux side, SuSE LVM has been in use for a long time.

However it is important to note that cluster volume management is different from the normal volume management. Cluster Volume management provides the volumes for all the nodes in the cluster infrastructure.


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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