Basic Differences Between Raw Device, ASM Device and CFS:
A traditional file system is a tree structure made up of directories and files that is implemented through the operating system kernel. The file system uses the concept of cache buffering, optimizing the number of times the operating system must access the disk. The file system takes control of input / output operations by freeing processes from this operation and allowing them to continue with other operations. The file system caches data to write until it has multiple data to process together, which improves system performance.
In an Oracle RAC architecture, two or more instances on separate servers would share the same file system since the storage is unique for both instances. However, the traditional file systems provided by Unix do not offer this feature; in other words, two Unix servers cannot share the same file system.
There are three options that Oracle proposes to implement a multi-instance system on single storage: raw devices, Cluster File System (CFS) and Automatic Storage Management (ASM). Next, a detail of each option.
What is the Raw Devices?
A raw device is a contiguous region of a disk accessed through an interface that provides “raw” access to the input/output system. By saying “raw” access, we are saying that it is not through the file system. On Unix, this interface is implemented with a character device. In this way, direct access is achieved between a process and a logical disk. In such an implementation, executing a write operation by a process will move the data to the device directly (not through the file system). Although this solution is very efficient (especially in a system with high input/output contention), it is the most complex of all the options from an administrative point of view.
What is the Cluster File System (CFS) in Oracle?
Clustered file systems (CFS) allow multiple servers to access the same file system. CFS solves the disadvantages and complexities of raw devices by providing a simpler solution for storage management. Oracle supports various types of CFS: Oracle Cluster File system (OCFS), Veritas Cluster File System (VCFS), IBM GPFS, Tru64 File System. The disadvantage of this solution is its cost since the alternatives provided by the various vendors will require their corresponding licenses. In the case of Oracle Cluster File System (OCFS), although it is free, it is only available for Linux and Windows environments.
What is the Automatic Storage Management (ASM) in Oracle?
Automatic Storage Management is the solution provided and chosen by Oracle that virtualizes access to storage by acting as an interface between an Oracle instance and the storage device that contains the data. ASM provides both the features of a file system and a volume manager. ASM is implemented as an Oracle instance that must be present in each of the nodes that make up the cluster. ASM is the most recommended solution by Oracle. Implementing ASM avoids the complexity of raw devices and avoids the economic cost of licensing the Cluster File System.
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