Chapter 12 File System Implementation.PPT

Chapter 12: File System Implementation File System Structure File System Implementation Directory Implementation Allocation Methods Free-Space Management Efficiency and Performance Recovery Log-Structured File Systems NFS File-System Structure File structure Logical storage unit Collection of related information File system resides on secondary storage (disks). File system organized into layers. File control block – storage structure consisting of information about a file. Layered File System A Typical File Control Block In-Memory File System Structures Virtual File Systems Virtual File Systems (VFS) provide an object-oriented way of implementing file systems. VFS allows the same system call interface (the API) to be used for different types of file systems. The API is to the VFS interface, rather than any specific type of file system. User program Kernel, VSF interface Schematic View of Virtual File System Directory Implementation Linear list of file names with pointer to the data blocks. simple to program time-consuming to execute Hash Table – linear list with hash data structure. decreases directory search time collisions – situations where two file names hash to the same location fixed size An improved implementation is a hash table with a linked list (entry) Collision free Can grow Allocation Methods An allocation method refers to how disk blocks are allocated for files: Contiguous allocation Linked allocation Indexed allocation Contiguous Allocation Each file occupies a set of contiguous blocks on the disk. Simple – only starting location (block #) and length (number of blocks) are required. Random access. Wasteful of space (dynamic storage-allocation problem). Files cannot grow. Contiguous Allocation of Disk Space Extent-Based Systems Many newer file systems (I.e. Veritas File System) use a modified contiguous allocation scheme. Extent-based file systems allocate disk blocks in extents. An extent is a contiguous block of disks. Extents are