The io-blk shared library implements a buffer cache that all filesystems inherit. The buffer cache attempts to store frequently accessed filesystem blocks in order to minimize the number of times a system has to perform a physical I/O to the disk.
Read operations are synchronous; write operations are usually asynchronous. When an application writes to a file, the data enters the cache, and the filesystem manager immediately replies to the client process to indicate that the data has been written. The data is then written to the disk.
Critical filesystem blocks such as bitmap blocks, directory blocks, extent blocks, and inode blocks are written immediately and synchronously to disk.
Applications can modify write behavior on a file-by-file basis. For example, a database application can cause all writes for a given file to be performed synchronously. This would ensure a high level of file integrity in the face of potential hardware or power problems that might otherwise leave a database in an inconsistent state.