96 lines
5.2 KiB
HTML
96 lines
5.2 KiB
HTML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<meta name="DC.Type" content="concept" />
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<meta name="DC.Title" content="Records, blocks, and formats" />
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<meta name="abstract" content="Read this information to help you understand records, blocks, and record formats of the tapes." />
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<title>Records, blocks, and formats</title>
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</head>
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<body id="recblock"><a name="recblock"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Records, blocks, and formats</h1>
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<div><p><span>Read this information to help you understand
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records, blocks, and record formats of the tapes.</span></p>
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<dl><dt class="dlterm">Records</dt>
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<dd>Records are logical mappings of the data on a tape. It typically maps
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directly to the records in a database file.</dd>
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<dt class="dlterm">Blocks</dt>
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<dd>Blocks are physical units of data on a tape. Blocks can include a record,
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part of a record, or multiple records.</dd>
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<dt class="dlterm">Record block format</dt>
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<dd>The record block format allows the system and user to interpret the data
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on a tape.</dd>
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</dl>
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<p>To understand records, blocks, and record block formats, you must know
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a few key terms:</p>
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<dl><dt class="dlterm">Block Descriptor Word (BDW)</dt>
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<dd>One or more logical records or record segments follow a block descriptor
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word (BDW) in a variable length block.</dd>
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<dt class="dlterm">Blocked records</dt>
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<dd>Blocking is the process of grouping records into blocks before the system
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writes records on a volume. A block consists of one or more logical records.
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Blocking conserves storage space on a volume by reducing the number of interblock
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gaps in the data set. This increases processing efficiency by reducing the
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number of I/O operations that are required to process the data set.</dd>
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<dt class="dlterm">Deblocked records </dt>
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<dd>One record exists per block.</dd>
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<dt class="dlterm">Fixed length </dt>
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<dd>The blocks on a tape have an exact (or fixed) length.</dd>
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<dt class="dlterm">Interblock gap</dt>
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<dd>Interblock gap is the physical gap on tape between two data blocks.</dd>
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<dt class="dlterm">Record Descriptor Word (RDW)</dt>
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<dd>Data follows a record descriptor word (RDW) in a variable length logical
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record. The RDW describes the record.</dd>
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<dt class="dlterm">Record Segment</dt>
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<dd>As spanned records occupy more than one block, each part of the record
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is a record segment.</dd>
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<dt class="dlterm">Segment Descriptor Word (SDW)</dt>
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<dd>Data follows a segment descriptor word (SDW) in each record segment. The
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SDW, similar to the RDW, describes the record segment.</dd>
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<dt class="dlterm">Spanned records</dt>
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<dd>The system splits a single record (spans) into two different data blocks
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and writes them on the tape.</dd>
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<dt class="dlterm">Undefined length</dt>
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<dd>The blocks on a tape have no defined length; each block can be different;
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and the program application interprets each block correctly.</dd>
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<dt class="dlterm">Unspanned records</dt>
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<dd>Each record is contained within one data block.</dd>
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<dt class="dlterm">Variable length</dt>
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<dd>The blocks on a tape have a variable length. The block contains a header
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with the length of the block. Each block in a file might or might
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not have the same length.</dd>
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</dl>
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<p>In sorting out these terms, the program supports and translates certain
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combinations into record block formats.</p>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="concept.htm" title="Find out how tape files operate on your server.">Concepts</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="formd.htm" title="Here are some examples of different record formats.">Record formats</a></div>
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</div>
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</div>
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</body>
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</html> |