第六章文件系统 高海昌.pptVIP

* Linux虚拟文件系统VFS 整个 Linux 系统通过 Virtual Filesystem Switch 的核心功能去读取 filesystem。整个 Linux 认识的 filesystem 都是 VFS 在进行管理，使用者并不需要知道每个 partition 的 filesystem 是什么 * File System Reliability (4) Bit maps used by the logical dumping algorithm Step 1: examine all the entries. For each modified file, its i-node is marked in the bitmap. Each dir is also marked. Step 2: unmark any dir that have no modified files or dir in them or under them. Step 3: dump all the dir that are marked for dumping. Step 4: the files marked are also dumped. * File System Reliability (5) Tricky issues 棘手之处: Since the free block list is not a file, it is not dumped and hence it must be reconstructed from scratch after all the dumps have been restored. If a file is linked to two or more dir, it is important that the file is restored only one time. UNIX files may contain holes. The blocks in between are not part of the file and should not be dumped and not be restored. Special files, named files, and the like should never be dumped. * File System Consistency File system states (a) Consistent (b) missing block (c) duplicate block in free list (d) duplicate data block If the system crashes before all the modified blocks have been written out, the file system can be left in an inconsistent state. * File System Performance Access to disk is much slower than access to memory. Many file system designed with various optimizations to improve performance: Caching Block read ahead Reducing disk arm motion * Caching The block cache data structures When a block has to be loaded into a full cache, some block has to be removed. (like paging, use LRU) By writing critical blocks quickly, we greatly reduce the probability that a crash will wreck the file system. * Caching It is undesirable to keep data blocks in the cache too long before writing them out. Systems take two approaches: UNIX, have a system call, sync, which forces all the modified blocks out onto the disk immediate