Oracle Tips by Burleson

Oracle10g Grid Computing with RAC
Chapter 5 - Preparing Shared Storage

Direct I/O

Buffered I/O uses precious resources like memory and CPU cycles because the Oracle blocks are cached both in the SGA and in the file system buffer cache. By adopting the Direct I/O, a much higher cache hit ratio can be achieved. Oracle can handle cache much more efficiently than a file system. Oracle has a sophisticated touch count-based cache replacement algorithm that is sensitive to both the frequency of usage and how recent the data blocks are.

Buffered I/O fills up the file system cache with Oracle Data, where as using the Direct I/O allows non-Oracle data to be cached in the file system much more efficiently. In addition, the buffered I/O generally involves large physical writes, such as temp file writes. These writes are performed and waited for in series, and therefore they cannot be merged in the device driver or lower layers of the stack, unless a write-back disk cache is in use. This means that a full rotational latency of the physical disk is sustained between every pair of component writes.

Redo Log file writes also suffer from another severe inefficiency if buffered I/O is used. Because redo writes address an arbitrary number of log blocks, and log blocks are small relative to the size of file system buffers, it is normal that the last log block of a redo write does not align with the end of a file system buffer. Therefore, unless the target file system block is already in cache, the operating system must first read that block from disk before the new redo can be copied into part of its file system buffer.

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