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Unit 3: File System Hierarchy

the end of the file and the file pointer. Similarly, a write appends to the end of the end of the file notes
and advances to the end of the newly written material (the new end of file). Such a file can be reset
to the beginning, and, on some systems, a program may be able to skip forward or backward n
records, for some integer n. This scheme is known as sequential access to a file. Sequential access
is based on a tape model of a file.
A sequential file may consist of either formatted or unformatted records. If the records are
formatted, you can use formatted I/O statements to operate on them. If the records are
unformatted, you must use unformatted I/O statements only. The last record of a sequential file
is the end-of-file record.
3.3.2 Direct access

Direct access is based on a disk model of a file. For direct access, the file is viewed as a numbered
sequence of block or records. A direct-access file allows arbitrary blocks to be read or written.
Thus, after block 18 has been read, block 57 could be next, and then block 3. There are no
restrictions on the order of reading and writing for a direct access file. Direct access files are of
great use for intermediate access to large amounts of information.
The file operations must be modified to include the block number as a parameter. Thus, we have
“read n”, where n is the block number, rather than “read next”, and “write n”, rather that “write
next”. An alternative approach is to retain “read next” and “write next” and to add an operation;
“position file to n” where n is the block number. Then, to effect a “read n”, we would issue the
commands “position to n” and then “read next”.
Not all OS support both sequential and direct access for files. Some systems allow only sequential
file access; others allow only direct access. Some systems require that a file be defined as sequential
or direct when it is created; such a file can be accessed only in a manner consistent with its
declaration.

Note Direct-access files support both formatted and unformatted record types. Both
formatted and unformatted I/O work exactly as they do for sequential files.

3.3.3 other access methods

Other access methods can be built on top of a direct-access method. These additional methods
generally involve the construction of an index for a file. The index contains pointers to the various
blocks. To find an entry in the file, the index is searched first and the pointer is then used to access
the file directly to find the desired entry. With a large file, the index itself may become too large
to be kept in memory. One solution is to create an index for the index file. The primary index file
would contain pointers to secondary index files, which would point to the actual data items.

Example: IBM’s indexed sequential access method (ISAM) uses a small master index that
points to disk blocks of a secondary index. The secondary index blocks point to the actual file
blocks. The file is kept sorted on a defined key. To find a particular item, we first make a binary
search of the master index, which provides the block number of the secondary index. This block
is read in, and again a binary search is used to find the block containing the desired record.
Finally, this block is searched sequentially. In this way, any record can be located from its key by
at most direct access reads.

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