MECH 2301 Lecture Notes - Lecture 36: Sequential Access, Random Access
22 May 2018
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MECH 2301 Lecture 36 Notes
Introduction
Tape Allocation
• The blocks in a linked free space list are stored in the order in which files are deleted.
• Since deleted files are placed at the end of the list, the data on those blocks will be
recoverable until the blocks are needed for reuse.
• Note that the data in deleted files is not truly deleted from disk unless special effort is
made to clear or scramble all the bits in the blocks used by the file.
• This is a potential security risk.
• Special software, called shredder software, is available for the purpose of truly deleting
files from disk in a way that they cannot be recovered.
• New systems are starting to offer this feature as secure delete.
• Tape allocation is simpler than disk allocation.
• The size of a block can be varied to fit the logical requirements of the file.
• It is usually not practical to reallocate space in the middle of a tape, so files that grow
must be rewritten.
• If necessary, the tape can be compacted
• It is usually easier to do so by copying the tape to a new tape.
• Tape blocks can be linked, but, in general, files are stored contiguously whenever
possible.
• Tape is convenient for sequential access, but not practical for random access.
• Random access files that are stored on tape are normally moved to disk as a whole
before use.
CD, DVD, and Flash Drive Allocation
• The file system used for various optical drives and a flash drive is similar to those found
on hard disks.
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