mirror of
https://github.com/HDFGroup/hdf5.git
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a901d24a8b
Purpose:
Renamed old "HDF5 User's Guide" as "HDF5 User's Guide, Release 1.4.5".
Added navigation bar link to new user's guide on HDF server.
Changed release tag line (in old UG only) back to
"Describes HDF5 Release 1.4.5, February 200 3"
Platforms tested:
IE 5
261 lines
12 KiB
HTML
261 lines
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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" -->
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<!--
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* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
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* Copyright by the Board of Trustees of the University of Illinois. *
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* All rights reserved. *
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* *
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* This file is part of HDF5. The full HDF5 copyright notice, including *
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* terms governing use, modification, and redistribution, is contained in *
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* the files COPYING and Copyright.html. COPYING can be found at the root *
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* of the source code distribution tree; Copyright.html can be found at the *
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* root level of an installed copy of the electronic HDF5 document set and *
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* is linked from the top-level documents page. It can also be found at *
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* http://hdf.ncsa.uiuc.edu/HDF5/doc/Copyright.html. If you do not have *
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* access to either file, you may request a copy from hdfhelp@ncsa.uiuc.edu. *
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* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
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-->
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<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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<a href="http://hdf.ncsa.uiuc.edu/HDF5/doc/UG/index.html">HDF5 User's Guide for Release 1.6</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 from Release 1.4.5:</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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<a href="http://hdf.ncsa.uiuc.edu/HDF5/doc/UG/index.html">HDF5 User's Guide for Release 1.6</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 from Release 1.4.5:</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 1.4.5, February 2003
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</address><!-- #EndLibraryItem --><!-- 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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