15-02-2013, 11:38 AM
INDEXED ALLOCATION
INDEXED ALLOCATION.pptx (Size: 1.3 MB / Downloads: 22)
ALLOCATION METHODS…
The direct access nature of disks allows us flexibility in the implementation of
files. But many cases occur, when many files are stored on the same disk. Here arises
a problem of disc allocation i.e. how to allocate the disc space to these files so that the
Space on the disc is utilized effectively and files can be accessed quickly. Three major
methods of disc allocation are :
Contiguous Allocation
Linked Allocation
Indexed Allocation
Each method has its own advantages and disadvantages.
Generally , a system use one particular type of allocation methods
for all files. Still, there are many systems [such as Data General’s
RDOS for its Nova line of computers] which support all three
methods.
INDEXED ALLOCATION…
Linked Allocation came in to solve the external fragmentation and size
declaration problem of Contiguous Allocation. But still it had a problem that is, in
the absence of FAT(File Allocation Table),linked allocation can not support efficient
direct access, since the pointers to the blocks are scattered with the blocks
themselves all over the disk and need to be retrieved in order.
To overcome this problem, emerges the role of INDEXED
ALLOCATION. Indexed Allocation solves this problem by bringing
all the pointers together into one location : the Index Block.
Multilevel Index
A Variant of the linked representation is to use a first level index block to point to a set of second level index block,which in turn points to the file blocks.
Operating System Uses the first level index block to point to second level block and that block finds the desired data block.
It can go to any level which depends on desired
maximum file size