Tuning the Storage Subsystem - Expert Oracle RAC Performance Diagnostics and Tuning - page 367

Oracle-Supported Access Types
Starting with Oracle Database 11g Release 2, Oracle RAC supports two types of storage for database files:

• Clustered file system (CFS)

• Automatic Storage Management (ASM)

■ the aSm clustered file system is available as a shared storage solution; however, it's not supported for
database files at this time.

Note

Clustered File System
A file system is a hierarchical tree of directories containing files implemented on raw device partitions. In a file
system, there is a cache buffer over the raw devices that optimizes the number of times the OS must access the disk.
This releases the process that actually writes to disk, freeing the process to perform its next write-to-cache operation.
Buffering helps retain data until such time as the buffer is full before performing a bulk write to the disk. This can have
the effect of enhancing system performance.
System failures before writing the data from the cache can result in the loss of file system integrity. Additionally,
the file system adds overhead to any operation that reads or writes data in direct accordance to its physical layout.
Clustered file systems (CFS), illustrated earlier in Figure 12-2 , allow access from multiple hosts to the same file
system data. This reduces the number of copies of the same data while distributing the load across those hosts going
to the same data.
When an application operates against a file, that action brings into play various pieces of file system metadata
that map a file to blocks of storage, which are typically accessed and managed using a file system cache.
CFS overcomes the administrative drawbacks of using a collection of raw devices and provides an easier storage
management solution. Oracle supports several types of file systems; for example, Oracle clustered file system (OCFS),
Veritas clustered file system (VCFS), Tru64 file system, and so on. These file systems have been popularly used on
their respective supported platforms—for example, VCFS is used on Sun clusters and more recently on AIX platforms;
OCFS is developed by Oracle Corporation and supports both the Windows and Linux environments.
Oracle also supports another file system maintaining the underlying ASM diskgroups—the ASM clustered file
system (ACFS). ACFS is discussed later in this section.

Automatic Storage Management
Oracle Automatic Storage Management (ASM) is a combined volume manager and cluster-capable file system. Disks
are allocated to ASM for management and control in the same way that a volume manager manages volumes. ASM is
highly integrated with, and highly optimized for, Oracle Database. It has become the best-practice standard for Oracle
Database storage.
Combining volume management functions with a file system (illustrated in Figure 12-9 Configuration type A)
allows a level of integration and efficiency that would not otherwise be possible. For example, ASM is able to avoid the
overhead associated with a conventional file system and achieve native raw disk performance for the files it supports.
ASM also supports the use of just a bunch of disks (JBOD) for the creation of a diskgroup. When individual disks
are used, multiple disks are combined together to form a diskgroup, as illustrated in Figure 12-9 Configuration type B.