Module 11 - File-system implementation. In this chapter we discuss various methods for storing information on secondary storage. The basic issues are device directory, free space management, and space allocation on a disk. | Lecture Operating system concepts Fifth edition Module 11 - Avi Silberschatz Peter Galvin File-System Implementation File-System Structure Allocation Methods Free-Space Management Directory Implementation Efficiency and Performance Recovery Silberschatz and Galvin 1999 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. Silberschatz and Galvin 1999 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. Mapping from logical to physical. Q LA 512 R Block to be accessed starting address Displacement into block R Silberschatz and Galvin 1999 Linked Allocation Each file is a linked list of disk blocks blocks may be scattered anywhere on the disk. block pointer Silberschatz and Galvin 1999 Allocate as needed link together . file starts at block 9 Silberschatz and Galvin 1999 Linked Allocation Cont. Simple need only starting address Free-space management system no waste of space No random access Mapping Q LA 511 R Block to be accessed is the Qth block in the linked chain of blocks representing the file. Displacement into block R 1 File-allocation table FAT disk-space allocation used by MS- DOS and OS 2. Silberschatz and Galvin 1999 Indexed Allocation Brings all pointers together into the index block. Logical view. index table Silberschatz and Galvin 1999 Example of Indexed Allocation Silberschatz and Galvin 1999 Indexed Allocation Cont. Need index table Random access Dynamic access without external fragmentation but have overhead of index block. Mapping from logical to physical in a file of maximum size of 256K words and block size of 512 .
