File Allocation Methods

There are three major methods of storing files on disks: contiguous, linked, and indexed.

1.      Contiguous Allocation

Contiguous Allocation requires that all blocks of a file be kept together contiguously. Performance is very fast, because reading successive blocks of the same file generally requires no movement of the disk heads, or at most one small step to the next adjacent cylinder.

Problems can arise when files grow, or if the exact size of a file is unknown at creation time:

o    Over-estimation of the file's final size increases external fragmentation and wastes disk space.

o    Under-estimation may require that a file be moved or a process aborted if the file grows beyond its originally allocated space.

o    If a file grows slowly over a long time period and the total final space must be allocated initially, then a lot of space becomes unusable before the file fills the space.

A variation is to allocate file space in large contiguous chunks, called extents. When a file outgrows its original extent, then an additional one is allocated. ( For example an extent may be the size of a complete track or even cylinder, aligned on an appropriate track or cylinder boundary. ) The high-performance files system Veritas uses extents to optimize performance.

2.      Linked Allocation

·         Disk files can be stored as linked lists, with the expense of the storage space consumed by each link. ( E.g. a block may be 508 bytes instead of 512. )

·         Linked allocation involves no external fragmentation, does not require pre-known file sizes, and allows files to grow dynamically at any time.

·         Unfortunately linked allocation is only efficient for sequential access files, as random access requires starting at the beginning of the list for each new location access.

·         Allocating clusters of blocks reduces the space wasted by pointers, at the cost of internal fragmentation.

·         Another big problem with linked allocation is reliability if a pointer is lost or damaged. Doubly linked lists provide some protection, at the cost of additional overhead and wasted space.

·         The File Allocation Table, FAT, used by DOS is a variation of linked allocation, where all the links are stored in a separate table at the beginning of the disk. The benefit of this approach is that the FAT table can be cached in memory, greatly improving random access speeds.

3.      Indexed Allocation

Indexed Allocation combines all of the indexes for accessing each file into a common block ( for that file ), as opposed to spreading them all over the disk or storing them in a FAT table.

Some disk space is wasted ( relative to linked lists or FAT tables ) because an entire index block must be allocated for each file, regardless of how many data blocks the file contains.

Performance

·         The optimal allocation method is different for sequential access files than for random access files, and is also different for small files than for large files.

·         Some systems support more than one allocation method, which may require specifying how the file is to be used ( sequential or random access ) at the time it is allocated. Such systems also provide conversion utilities.

·         Some systems have been known to use contiguous access for small files, and automatically switch to an indexed scheme when file sizes surpass a certain threshold.

·         And some systems adjust their allocation schemes ( e.g. block sizes ) to best match the characteristics of the hardware for optimum performance.