Block I/O

　　

The storage media contains files residing in a filesystem, such as EXT3 or Reiserfs. User applications invoke I/O system calls to access these files. The resulting filesystem operations pass through the generic Virtual File System (VFS) layer before entering the individual filesystem driver. The buffer cache speeds up filesystem access to block devices by caching disk blocks. If a block is found in the buffer cache, the time required to access the disk to read the block is saved. Data destined for each block device is lined up in a request queue. The filesystem driver populates the request queue belonging to the desired block device, whereas the block driver receives and consumes requests from the corresponding queue. In between, I/O schedulers manipulate the request queue so as to minimize disk access latencies and maximize throughput.