36 lines
1.6 KiB
Plaintext
36 lines
1.6 KiB
Plaintext
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Database structures.
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Most databases have a structure with a header record. The header record
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is at the beginning of the datafile and contains information about the
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size of the header record and size of the database records. When a data
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file is opened for reading the first thing to read the header record.
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The field recsize contains the size of the datarecords and the field
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hdrsize the offset to the first datarecord in the file.
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If in the structure the size of the datarecords changes (grows), we can
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allways read the old format in the correct way.
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When a datafile is changed the datafile has to be rewritten completly.
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Of course the new format is used then, and the new size must be stored in
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the header.
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The advantage of this technique is that updates can be performed automatic.
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There is no need for free space for future use in the datarecords, the files
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are thus smaller.
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One other important thing, with some DOS based bbs'es, mail/tic processors
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are using index files together with the data files to speed up the search in
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the databases. Also some of them use internal memory cache for the data records.
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I choose not to do this for two reasons, Linux like other Unices handles
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file I/O very fast and when your system is not low on memory the kernel will
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buffer all disk I/O in memory. Also Linux disks are very low fragmented due to
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the design of the ext2fs. Whith all this in mind, using index files is only
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extra overhead.
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However, because of this you should not put the data files on a msdos
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dos partition or on a nfs server.
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The only exeption that uses index files are the nodelists.
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