unbuffered I/O

I/O that bypasses the file system cache for the purpose of increasing I/O performance (also known as direct I/O).

I/O that bypasses the kernel cache to increase I/O performance. This is similar to direct I/O, except when a file is extended; for direct I/O, the inode is written to disk synchronously, for unbuffered I/O, the inode update is delayed.

See buffered I/O and direct I/O.