File-System Structure

Transcription

1 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 12.1 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 with of information about a file 12.2 Layered File System

2 A Typical File Control Block 12.4 File System Access example 12.5 Virtual File Systems Virtual File Systems (VFS) provide an objectoriented 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

3 Schematic View of Virtual File System 12.7 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 issues: collisions: several file names may hash to same location fixed size 12.8 Allocation Methods how disk blocks are allocated for files: Contiguous allocation Linked allocation Indexed allocation

4 Contiguous Allocation Each file occupies set of contiguous blocks on disk simple only starting location (block #) and length (number of blocks) are required random access external fragmentation files cannot grow Contiguous Allocation of Disk Space Extent-Based Systems modified contiguous allocation disk blocks allocated in contiguous extents An extent is a contiguous block of disks Extents are allocated for file allocation A file consists of one or more extents

5 Linked Allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk each block contains index of next block Simple need only starting address Free-space management system no waste of space No random access Logical block size reduced Linked Allocation File-Allocation Table linked-allocation scheme used in MSDOS cached in memory

6 Indexed Allocation Brings all pointers together into the index block Indexed Allocation Needs extra space for index block small files: large overhead large files: multiple index blocks Supports random access No external fragmentation Multi-level Indexed Allocation M outer-index index table file

7 Combined Scheme: UNIX (4KB blocks) Free-Space Management Bit vector Linked list Grouping Counting Bit Vector Maintain bit vector reflecting block usage n-1 bit[i] = block[i] free 1 block[i] occupied

8 Linked Free Space List on Disk Free Space Issues Bit map requires extra space: block size = 2 12 bytes disk size = 2 30 bytes (1 gigabyte) n = 2 30 /2 12 = 2 18 bits (or 32K bytes) Easy to get contiguous files Linked list (free list) Cannot get contiguous space easily No waste of space Grouping index block of free blocks Counting store address of free block plus count free contiguous blocks Efficiency and Performance Efficiency depends on: disk allocation and directory algorithms types of data kept in file s directory entry Performance disk cache separate section of main memory for frequently used blocks free-behind and read-ahead techniques to optimize sequential access improve PC performance by dedicating section of memory as virtual disk, or RAM disk

9 Various Disk-Caching Locations Double Buffering I. memory-mapped I/O uses page cache II. routine I/O via file system uses the disk buffer cache Double Buffering better: unified buffer uses page cache for both memory-mapped and routine file system I/O fast: hardware supported

10 Recovery Consistency checking compares data in directory structure with data blocks on disk: tries to fix inconsistencies back up copy disk data to another storage device (floppy disk, magnetic tape) Recover lost file or disk by restoring data from backup Log-Structured File Systems also: journaling file system record each update to the file system as a transaction all transactions are written to a log transaction is considered committed once it is written to the log transactions in log are asynchronously written to the file system when file system is modified: transaction is removed from log after system crash: all remaining transactions in log must still be performed