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File control block storage structure consisting of

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Unformatted text preview: esides on secondary storage (disks). File control block – storage structure consisting of information information about a file (kept within directory) User issues a file logical address LA … turned into physical physical disk address Typical disk block is 4K – disk operations allow random seek File File-System Structure File size constrained by address range of machine; a 32 32-bit file pointer/offset can address 4 GB only - though on some filesystems, files can exceed this OS OS maintains free disk blocks (also stored on the disk itself) Contiguous Contiguous Allocation Each Each file occupies a set of contiguous blocks/sectors on the disk. Simple – only starting location (disk block #) and length (number (number of blocks or bytes) are required. Accessing block b+1 after block b normally requires no disk head movement modulo disk scheduling algorithm (BIG win) Example Example Contiguous Contiguous Allocation Problems Linked Linked Allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. disk block = pointer data Allocate as needed, link together; e.g., file starts at block 9 count directory: contiguous File start length count 2 2 Linked Linked Allocation (Cont.) Simple – need only starting address Free-space management – less waste of space Files Files can grow Problems: Linked Linked Allocation (Cont.) File Allocation Table (FAT) stored on disk (MS-DOS, OS-2) directory name Start block 217 217 339 indexed by physical block 618 618 End-of-file 339 -1 Grow: look for first 0 entry in FAT Random access improved if cached in memory (read the FAT … stored contiguously at well-known location on disk). Could have two seeks per block unless cached in memory. Problems Problems with FAT FAT is like a big page table 40 GB disk and 4K disk blocks-> 40 MB of space Break Break Indexed Indexed Allocation Want efficient random access with pointer blocks, and more memory efficient Brings all pointers...
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