A file is simply a collection of data stored on media. When a process wants to access data from a file, the operating system brings the data into main memory, the process reads it, alters it and stores to the disk . The operating system could read and write data directly to and from the disk for each request, but the response time and throughput would be poor due to slow disk access times. The operating system therefore attempts to minimize the frequency of disk accesses by buffering data in main memory, within a structure called the file buffer cache.
Certain applications derive no benefit from the file buffer cache. Databases normally manage data caching at the application level, so they do not need the file system to implement this service for them. The use of a file buffer cache results in undesirable overheads in such cases, since data is first moved from the disk to the file buffer cache and from there to the application buffer. This “doublecopying” of data results in more CPU consumption and adds overhead to the memory too.
For applications that wish to bypass the buffering of memory within the file system cache, Direct I/O is provided. When Direct I/O is used for a file, data is transferred directly from the disk to the application buffer, without the use of the file buffer cache. Direct I/O can be used for a file either by mounting the corresponding file system with the direct i/o option (options differs for each OS), or by opening the file with the O_DIRECT flag specified in the open() system call. Direct I/O benefits applications by reducing CPU consumption and eliminating the overhead of copying data twice – first between the disk and the file buffer cache, and then from the file.
However, there are also few performance impacts when direct i/o is used. Direct I/O bypasses filesystem read-ahead – so there will be a performance impact.
