Linux File System: Raw IO and Direct IO

Raw IO and Direct IO are both bypassing file system cache. Here are more details about these two IOs. This can help us understand the difference and troubleshoot easier when we get a file system issue.

Difference Between RAW IO and Direct IO

Raw I/O is issued directly to disk offsets, bypassing the file system altogether. It has been used by some applications, especially databases, that can manage and cache their own data better than the file system cache. A drawback is more complexity in the software. From Oracle’s official website, input/output (I/O) to a raw partition offers approximately a 5% to 10% performance improvement over I/O to a partition with a file system on it.

Direct I/O allows applications to use a file system but bypass the file system cache, for example, by using the O_DIRECT open(2) flag on Linux. This is similar to synchronous writes (but without the guarantees that O_SYNC offers), and it works for reads as well. It isn’t as direct as raw device I/O, since mapping of file offsets to disk offsets must still be performed by file system code, and I/O may also be resized to match the size used by the file system for on-disk layout (its record size) or it may error (EINVAL).