mirror of
https://github.com/HDFGroup/hdf5.git
synced 2024-11-27 02:10:55 +08:00
d586a1adb9
Purpose:
DreamWeaver (DW) adaptation of the HDF5 doc set, introducing the use of
stylesheets and DW libraries.
Stylesheets -- for consistent format management.
DW libraries -- for managing common elements across documents.
Platforms tested:
IE 5 and 6
Netscape
244 lines
10 KiB
HTML
244 lines
10 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
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<html>
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<head>
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<title>Performance</title>
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<!-- #BeginLibraryItem "/ed_libs/styles_UG.lbi" --><link href="ed_styles/UGelect.css" rel="stylesheet" type="text/css">
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<!-- #EndLibraryItem --></head>
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<body bgcolor="#FFFFFF">
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<!-- #BeginLibraryItem "/ed_libs/NavBar_UG.lbi" --><hr>
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<center>
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<table border=0 width=98%>
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<tr><td valign=top align=left>
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<a href="index.html">HDF5 documents and links</a> <br>
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<a href="H5.intro.html">Introduction to HDF5</a> <br>
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<a href="RM_H5Front.html">HDF5 Reference Manual</a> <br>
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<!--
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<a href="Glossary.html">Glossary</a><br>
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-->
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</td>
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<td valign=top align=right>
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And in this document, the
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<a href="H5.user.html"><strong>HDF5 User's Guide:</strong></a>
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<br>
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<a href="Files.html">Files</a>
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<a href="Datasets.html">Datasets</a>
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<a href="Datatypes.html">Datatypes</a>
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<a href="Dataspaces.html">Dataspaces</a>
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<a href="Groups.html">Groups</a>
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<br>
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<a href="References.html">References</a>
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<a href="Attributes.html">Attributes</a>
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<a href="Properties.html">Property Lists</a>
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<a href="Errors.html">Error Handling</a>
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<br>
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<a href="Filters.html">Filters</a>
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<a href="Caching.html">Caching</a>
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<a href="Chunking.html">Chunking</a>
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<a href="MountingFiles.html">Mounting Files</a>
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<br>
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<a href="Performance.html">Performance</a>
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<a href="Debugging.html">Debugging</a>
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<a href="Environment.html">Environment</a>
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<a href="ddl.html">DDL</a>
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</td></tr>
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</table>
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</center>
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<hr>
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<!-- #EndLibraryItem --><h1>Performance Analysis and Issues</h1>
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<h2>1. Introduction</h2>
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<p>This section includes brief discussions of performance issues
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in HDF5 and performance analysis tools for HDF5 or pointers to
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such discussions.
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<h2>2. Dataset Chunking</h2>
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Appropriate dataset chunking can make a siginificant difference
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in HDF5 performance. This topic is discussed in
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<a href="Chunking.html">Dataset Chunking Issues</a> elsewhere
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in this <cite>User's Guide</cite>.
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<a name="Freespace">
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<h2>3. Freespace Management</h2>
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</a>
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<p>HDF5 does not yet manage freespace as effectively as it might.
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While a file is opened, the library actively tracks and re-uses
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<em>freespace</em>, i.e., space that is freed (or released)
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during the run.
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But the library does not yet manage freespace across the
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closing and reopening of a file; when a file is closed,
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all knowledge of available freespace is lost.
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What was freespace becomes an unusable <em>hole</em> in the file.
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<p>There are several circumstances that can result in freespace
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in an HDF5 file:
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<ul>
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<li>Reading then rewriting a dataset or compressed dataset
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chunk.<sup><a href="#footcchunk">1</a></sup>
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<ul>
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<li>If the rewritten dataset or compressed chunk is the same
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size as or smaller than the original, it will be written
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to the same file location.
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<li>If, however, the dataset or compressed chunk is larger
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than the original, it will be written contiguously elsewhere
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in the file, leaving freespace at the original location.
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<li>If the rewritten dataset or compressed chunk is
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substantially smaller than the original, the remaining
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space will be released and identified as freespace.
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</ul>
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<li>Deleting (or unlinking) a dataset or group.
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<ul>
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<li>If an object, such as a dataset, group, or named datatype,
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is deleted (normally with <code>H5Gunlink</code>),
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the space previously occupied by the object is released
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and identified as freespace.
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</ul>
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</ul>
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<p>As stated above, freespace is not managed across the
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closing and reopening of an HDF5 file; file space that was
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known freespace while the file remained open becomes an
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inaccessible hole when the file is closed.
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Thus, if a file is often closed and reopened, datasets
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frequently rewritten, or groups and/or datasets frequently
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added and deleted, that file can develop large numbers of
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holes and grow unnecessarily large. This can, in turn,
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seriously impair application or library performance
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as the file ages.
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<p>An <code>h5pack</code> utility would enable <em>packing</em>
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a file to remove the holes, but writing such a utility to
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universally pack the file correctly is a complex task and the
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HDF5 development team has not to date had the resources to
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complete the task.
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<p>For application developers or researchers who find themselves
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working with files that become bloated in this manner, there
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are, at this time, two remedies:
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<ul>
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<li><code>H5view</code>, an HDF5 Java tool, allows the user
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to open a file and, using the <code>Save As...</code> feature,
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save the file under a new filename. The new file can then
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be closed and will be a packed version of the original file.
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This approach is reasonably reliable, but with two caveats:
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<ul>
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<li>It is not automated.
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<li>This ability is a side-effect of the tool's design;
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it was not designed for this purpose and this approach
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to file packing has not been exhaustively tested.
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</ul>
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<li>An application developer or researcher can write a utility
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that is tuned to their data and file structures. This
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untility can then read in a file, copy the structures and
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datasets to a new file, and write the new file to storage.
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This will eliminate the holes, making the new file a
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fully-packed version of the original file.
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</ul>
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<a name="footcchunk">
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<p></a>
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<sup>1</sup>
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<font size=-1>
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This is a problem only with compressed chunks.
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The compression ratio of data is highly dependent on the data
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itself; regardless of whether the <em>size</em> of the data
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changes, the size of the compressed data change substantially
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as the data changes. Uncompressed chunks do not vary in size,
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so this issue does not arise.
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</font>
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<h2>4. Use of the Pablo Instrumentation of HDF5</h2>
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Pablo HDF5 Trace software provides a means of measuring the
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performance of programs using HDF5.
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<p>The Pablo software consists
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of an instrumented copy of the HDF5 library, the Pablo Trace and
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Trace Extensions libraries, and some utilities for processing the
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output. The instrumented version of the HDF5 library has hooks
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inserted into the HDF5 code which call routines in the Pablo Trace
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library just after entry to each instrumented HDF5 routine and
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just prior to exit from the routine. The Pablo Trace Extension
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library has programs that track the I/O activity between the
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entry and exit of the HDF5 routine during execution.
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<p>A few lines of code must be inserted in the user's main program
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to enable tracing and to specify which HDF5 procedures are to be
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traced. The program is linked with the special HDF5 and Pablo
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libraries to produce an executable. Running this executable on
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a single processor produces an output file called the trace file
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which contains records, called Pablo Self-Defining Data Format
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(SDDF) records, which can later be analyzed using the
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HDF5 Analysis Utilities. The HDF5 Analysis Utilites can be used
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to interpret the SDDF records in the trace files to produce a
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report describing the HDF5 IO activity that occurred during
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execution.
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<p>For further instructions, see the file <code>READ_ME</code>
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in the <code> $(toplevel)/hdf5/pablo/ </code> subdirectory of
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the HDF5 source code distribution.
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<p>For further information about Pablo and the
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Self-Defining Data Format, visit the Pablo website at
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<code><a href="http://www-pablo.cs.uiuc.edu/">http://www-pablo.cs.uiuc.edu/</a></code>.</p>
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<!-- #BeginLibraryItem "/ed_libs/NavBar_UG.lbi" --><hr>
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<center>
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<table border=0 width=98%>
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<tr><td valign=top align=left>
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<a href="index.html">HDF5 documents and links</a> <br>
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<a href="H5.intro.html">Introduction to HDF5</a> <br>
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<a href="RM_H5Front.html">HDF5 Reference Manual</a> <br>
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<!--
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<a href="Glossary.html">Glossary</a><br>
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-->
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</td>
|
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<td valign=top align=right>
|
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And in this document, the
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<a href="H5.user.html"><strong>HDF5 User's Guide:</strong></a>
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<br>
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<a href="Files.html">Files</a>
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<a href="Datasets.html">Datasets</a>
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<a href="Datatypes.html">Datatypes</a>
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<a href="Dataspaces.html">Dataspaces</a>
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<a href="Groups.html">Groups</a>
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<br>
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<a href="References.html">References</a>
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<a href="Attributes.html">Attributes</a>
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<a href="Properties.html">Property Lists</a>
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<a href="Errors.html">Error Handling</a>
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<br>
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<a href="Filters.html">Filters</a>
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<a href="Caching.html">Caching</a>
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<a href="Chunking.html">Chunking</a>
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<a href="MountingFiles.html">Mounting Files</a>
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<br>
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<a href="Performance.html">Performance</a>
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<a href="Debugging.html">Debugging</a>
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<a href="Environment.html">Environment</a>
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<a href="ddl.html">DDL</a>
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</td></tr>
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</table>
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</center>
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<hr>
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<!-- #EndLibraryItem --><!-- #BeginLibraryItem "/ed_libs/Footer.lbi" --><address>
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<a href="mailto:hdfhelp@ncsa.uiuc.edu">HDF Help Desk</a>
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<br>
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Describes HDF5 Release 1.5, Unreleased Development Branch
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</address><!-- #EndLibraryItem -->
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<!-- Created: Thu Oct 14 16:46:00 CDT 1999 -->
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<!-- hhmts start -->
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Last modified: 2 August 2001
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<!-- hhmts end -->
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</body>
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</html>
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