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Donald K. Burleson

Oracle RAC Tips

Cluster File System

A cluster file system provides an alternative to raw devices. A file system presents a familiar environment to the user for creating and managing the files required for a RAC database. The cluster file system provides a view of the database files and makes it easy to expand, add, and copy existing files. With a shared file system, multiple instances can share archive logs. This makes media recovery much more convenient because every node has access to every archived log file as needed. The general features include:

  • For applications, the CFS functionality is just like a regular file system. The RDBMS system is easily integrated with the CFS and requires no special modifications.

  • Most of the CFS solutions are based on a master-server design, where one of the nodes acts as a server and the others act as clients. The master node performs all the file system metadata manipulations, with other nodes forwarding the tasks to the server node (e.g., Veritas CFS).

  • Polyserve Matrix Server is based on symmetric architecture with a distributed lock manager. This means that all the nodes in the cluster collectively undertake the metadata and lock management operations. No bottleneck and no single point of failure.

  • When using a CFS, Oracle9i RAC requires Direct I/O. Direct I/O is generally enabled for a file by opening the file with the o_direct flag in the open(2) system call.. Read and write requests for the file are then executed to and from disk storage through direct memory access thus bypassing CFS caching. This improves the I/O performance for database applications that perform their own caching and file region synchronization. This is, however, a compile-time requirement. To that end, PolyServe Matrix Server features a mount-time Direct I/O that enables any application that is capable of using raw partitions to benefit from Direct I/O without recompiling.

  • Without the Direct I/O feature (or the ODM interface), maintenance and synchronization of the local file system cache becomes complicated. A typical CFS provides an internal locking mechanism to control the concurrent use and protect the integrity of the file system (metadata). A locking mechanism, usually called the lock manager, is used for metadata integrity and cache coherency across the multiple nodes. The lock manager provides a way to ensure all nodes have a consistent view of the file system. The lock manager can be of distributed or master-slave architecture.

  • One of the advantages that accrue from using the cluster file system is the combination of OMF with the ODM interface. The cluster file systems that are compliant with the ODM interface are able to provide much better I/O performance, with relative ease of administration. Oracle managed files are able to create and delete files directly at the operating system level. We will discuss OMF and ODM again later.

For more information, see the book Oracle 11g Grid and Real Application Clusters 30% off if you buy it directly from Rampant TechPress . 

Written by top Oracle experts, this RAC book has a complete online code depot with ready to use RAC scripts.






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