Chapter 11: File-System Implementation

Transcription

1 Chapter 11: File-System Implementation

2 Chapter 11: 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 Operating Systems CS

3 Objectives To describe the details of implementing local file systems and directory structures To discuss block allocation and free-block algorithms and trade-offs Operating Systems CS

4 File-System Structure File structure Logical storage unit Collection of related information File system resides on secondary storage (disks). Allows read, write and modify block of data from disk Sequence or Random access to any given block of information I/O transfers between disk and memory in blocks of typically 512 bytes (efficiency) File system organized into layers Efficiency Avoids duplication of code File System maintains file structure via File control block: storage structure consisting of information about a file Ownership Permissions Locations of file contents Operating Systems CS

5 File system Design Challenges The file system provides efficient and convenient access to information on the disk File System Design Challenges: 1. File system UI File Attributes, Operations Directory structure 2. Mapping logical file system to physical storage (disk) Requires efficient algorithms and data structures Operating Systems CS

6 File System Layered Design In Layered Architecture Higher levels make use of features in the lower level I/O Control Uses Device drivers to transfer info between memory and disk Device driver Translates commands to hardware instructions for use by controller Basic file system: Issues generic commands to read/write physical blocks on disk File Organization Translates logical block addresses to physical block addresses Logical File System Manages directory structure Metadata information Maintains file structure via File Control Block (FCB) Operating Systems CS

7 A Typical File Control Block Operating Systems CS

8 Creating a new file 1. Your application program calls the logical file system 2. Logical file system creates a new FCB (allocates one from free FCBs) 3. Systems reads appropriate directory into memory Creates new file name Creates new FCB 4. Writes back onto disk Now the new file may be used for I/O operations Operating Systems CS

9 In-Memory File System Structures Necessary file system structures provided by the OS. Figure 12-3(a) refers to opening a file. Figure 12-3(b) refers to reading a file. Operating Systems CS

10 In-Memory File System Structures The open( ) call first searches systemwide-open file table to see if file is in use If file is in use: An entry is created in per-process open-file table. Entry points to system-wide open-file-table Else: Search directory structure for fie name Copy file s FCB to system-wide open file table An entry is created in per-process open-file table. Entry points to system-wide open-file-table Open( ) call returns a pointer to the entry in the per-process open-file table All I/O on the file are performed via pointer Operating Systems CS

11 Virtual File Systems OS concurrently supports multiple type of file-systems via VFS interface Virtual File Systems (VFS) provide an object-oriented way to simplify, organize and modularize File-system implementation VFS provides a uniform 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: Using data structures and procedures Operating Systems CS

12 Schematic View of Virtual File System Layer 1: File System Interfaces handles: System calls: open(), read(), write(), close() Layer 2: Virtual File System Separates generic operations from implementation details API provides a uniform interface to different types of file system implementations Layer 3: Supported file system types Operating Systems CS

13 Directory Implementation Directory management algorithms can affect efficiency of file-systems Linear list simple to program but time-consuming to execute Maintain a list of file names with pointers to the data blocks. To create a new file: Search dir (or list) to be sure file does not exist Add new at end of list To delete a file: Search list for file then delete name from file Disadvantage: Linear search of directory is time consuming Hash Table linear list with hash data structure. Linear list stores directory entries Harsh table takes a value computed from file name and returns a pointer to file name in the list decreases directory search time collisions situations where two file names hash to the same location Disadvantage: Fixed size Operating Systems CS

14 Allocation Methods An allocation method refers to how disk blocks are allocated for files: Contiguous allocation Linked allocation Indexed allocation Operating Systems CS

15 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 Operating Systems CS

16 Contiguous Allocation Mapping from logical to physical LA/512 Block to be accessed =! + starting address Displacement into block = R Q R Operating Systems CS

17 Contiguous Allocation of Disk Space Operating Systems CS

18 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 allocated for file allocation A file consists of one or more extents. Operating Systems CS

19 Linked Allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. block = pointer Operating Systems CS

20 Linked Allocation (Cont.) Simple need only starting address Free-space management system no waste of space No random access Mapping LA/511 Q R Block to be accessed is the Qth block in the linked chain of blocks representing the file. Displacement into block = R + 1 Operating Systems CS

21 Linked Allocation Operating Systems CS

22 Indexed Allocation Brings all pointers together into the index block. Logical view. index table Operating Systems CS

23 Example of Indexed Allocation Operating Systems CS

24 Linked Allocation File-Allocation Table (FAT) MS-Dos and OS2 Operating Systems FAT resides at beginning of each disk partition Each FAT index represents block number of the file Last block has eof value Directory entry contains the block number of first block Each FAT entry contains the next block number of the file Operating Systems CS

25 Indexed Allocation Small Files (max size 256K words; block size 512 words) Recall, index allocation: uses directory entry table Uses Index table, size normally one disk block (512) Provides random access Provides dynamic access without external fragmentation, but have overhead of index block. For small files: Mapping from logical to physical: Requires only 1 index table Q = displacement into index table R = displacement into block LA/512 Q R Operating Systems CS

26 Indexed Allocation Large Files (Unbounded Length; block size 512 words) For Large Files: Linked scheme Link several blocks of index table (no limit on size). Q 1 = block of index table R 1 is used as follows: LA / (512 x 511) Q 1 R 1 Last word is pointer to another index block R 1 / 512 Q 2 R 2 Q 2 = displacement into block of index table R 2 displacement into block of file: Operating Systems CS

27 Indexed Allocation Two-Level Index Two-level index (maximum file size is ) LA / (512 x 512) Q 1 R 1 Q 1 = displacement into outer-index R 1 is used as follows: R 1 / 512 Q 2 R 2 Q 2 = displacement into block of index table R 2 displacement into block of file: Operating Systems CS

28 Indexed Allocation Multi-level Index Directory entry table points to the start of the outer index table Outer-index table points to a set of second level index blocks Second level block may also point to another set of third level index blocks Third level points to fourth Entry in index table contains direct access block address M outer-index index table file Operating Systems CS

29 Combined Scheme: UNIX File Systems (4K bytes per block) Inode contains N pointers of the index block (E.g., N =12) First nine pointers point to direct blocks (small files ~ 4*9 KB) The 10 th pointer points to an indirect block (an index block) Index block contains address of blocks that contain data blocks The 11 th pointer points to double indirect block (an index block) The double indirect block contains? The 12 th pointer points to triple indirect block Operating Systems CS

30 Free-Space Management Bit Vector To create new files, we need to re-use free space from deleted files How do you keep track of free space on the disk? Maintain a free-space list Space not allocated to files or directory Implemented as Bit Vector n-bit vector: n bit[i] = Block number calculation 0 block[i] free 1 block[i] occupied (number of bits per word) * (number of 0-value words) + offset of first 1 bit Block number calc applies to First non-zero word Operating Systems CS

31 Free-Space Management n-bit Vector Storage n-bit vector requires extra space Example: block size = 2 12 bytes disk size = 2 30 bytes (1 gigabyte) n = 2 30 /2 12 = 2 18 bits (or 32K bytes) Requires 32K bytes to store bit maps Easy to get contiguous files Operating Systems CS

32 Free-Space Management Linked Free Space List on Disk Basic idea is to link all free disk blocks Requires a pointer to first free block Pointer stored on disk and cache Each block contains a pointer to next free disk block Linked list (free list) Cannot get contiguous space easily No waste of space Operating Systems CS

33 Efficiency and Performance Efficiency dependent on: Disk allocation and directory algorithms Linked allocation Vs Index allocation Types of data kept in file s directory entry Pointer size limits the length of file 16-bit pointer 2 16 (64 KB) 32-bit pointer 2 32 (4 GB) Requires more disk space to store index tables Performance Disk cache separate section of main memory for frequently used Disk blocks Free-behind and read-ahead techniques to optimize sequential access Operating Systems CS

34 Performance Page Cache A page cache caches pages rather than disk blocks using virtual memory techniques Memory-mapped I/O uses a page cache Routine I/O through the file system uses the buffer (disk) cache This leads to the following figure Operating Systems CS

35 Performance I/O Without a Unified Buffer Cache Operating Systems CS

36 Performance Unified Buffer Cache A unified buffer cache uses the same page cache to cache both memory-mapped pages and ordinary file system I/O Operating Systems CS

37 Performance I/O Using a Unified Buffer Cache Operating Systems CS