Usually not provided by operating system. TLB is one example of a keyed search. The structure used to describe which sectors represent a file is called the file descriptor . Keep a free list of unused areas of the disk. When creating a file, make the user specify its length, allocate all the space at once.
Descriptor contains location and size.
With interleaved user requests, still can't eliminate all seeks. In each block of file keep pointer to next block. Can also keep a linked list of free blocks for the free list. Lots of seeking, even in sequential access. Allocate an array to hold pointers to all the blocks, but don't allocate the blocks.