MECH 2301 Lecture 31: MECH 2301 Lecture 31 Notes
22 May 2018
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MECH 2301 Lecture 31 Notes
Introduction
Indexed Allocation
• The FAT technique appears to have reached the end of its useful life
• Because it is still offered sometimes as an option when formatting devices, it is useful to
understand the method and its shortcomings.
• Indexed allocation is similar to linked allocation, with one major difference: the link
pointers for a file are all stored together in one block, called an index block.
• There is a separate index block for each file.
• Loading the index block into memory when the file is opened makes the link pointers
available at all times for random access.
• Assuming the same linkages shown, the index blocks would look like those
• Since the only index blocks in memory are those for open files, indexed allocation
represents a much more efficient use of memory.
• One method used by some systems, including recent versions of Windows, to reduce
memory usage even further is to allocate storage in groups of contiguous blocks as
much as possible.
• Rather than store individual block links, this method allows the system to store a single
link to the starting block of each group, together with a count of the number of blocks in
the group.
• For files that grow in size gradually this may not be a useful strategy
• Even for a file of known size that is to be stored on a disk that does not have a single
contiguous space that is sufficient for the entire file
• It may be possible to store the file in a small number of groups.
• Some modern file systems offer excess capacity when the file is created to allow for
future growth in the size of the file.
• Although this would have been considered wasteful in earlier days, the large capacity of
modern disks makes this solution feasible and practical today.
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