If Direct I/O is activated, the operating system's file system cache is bypassed. Because the I/O is not processed via the file system cache, the CPU requirement for the I/O request is reduced. Since Oracle databases have very good buffer cache management, this is usually exactly the desired behavior. In practice, it makes more sense to provide the database instance with more main memory, because the database simply knows much more precisely which blocks are really needed. There are exceptions to this, however. The buffer cache is not used in some Oracle processing - especially in parallel processing that uses FTSs. You can recognize this in Oracle from the I/O statistics "db file parallel read" and "db file parallel write". In the case of I/Os in sorts, there are the I/O statistics "Direct path reads" and "Direct path writes".
special features
If the files are in the ASM, files and always uses SETALL. Nevertheless iraq telegram data you should set the parameter: for example, if you write a database backup to a file system, the setting for the parameter applies to writing the backup sets.
If database files are on NFS, there is no file system cache on the server, so it is important that you always use Direct I/O here. You should also configure Oracle dNFS (dNFS always uses SETALL, but you should still configure it if Oracle needs to make a fallback to the OS NFS stack).
How should one FILESYSTEMIO_OPTIONSadjust now?
In 95% of cases, the setting FILESYSTEMIO_OPTIONS= SETALL is optimal with a correspondingly configured buffer cache. There is, however, an exception to this rule. If you find significantly more direct path I/Os in the statistics compared to "db file sequential read" / "db file scattered reads", you can try to reduce the buffer cache in favor of the file system cache and FILESYSTEMIO_OPTIONSset it to ASYNCH. Alternatively, you can tell the Oracle Optimizer to always use the buffer cache for parallel queries by adding the CACHE attribute to the tables. However, this can lead to the buffer cache contents not being available for other - non-parallelized - statements, which makes them slower.