mirror of
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Purpose:
Update "Describes HDF5 Release..." footer from R1.4 to
"Release 1.5, Unreleased Development Branch"
Platforms tested:
IE 5
178 lines
6.4 KiB
HTML
178 lines
6.4 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
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<html>
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<head>
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<title>Data Caching</title>
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</head>
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<body bgcolor="#FFFFFF">
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<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="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="index.html">Other HDF5 documents and links</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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Caching
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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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<h1>Data Caching</h1>
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<h2>1. Meta Data Caching</h2>
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<p>The HDF5 library caches two types of data: meta data and raw
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data. The meta data cache holds file objects like the file
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header, symbol table nodes, global heap collections, object
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headers and their messages, etc. in a partially decoded
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state. The cache has a fixed number of entries which is set with
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the file access property list (defaults to 10k) and each entry
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can hold a single meta data object. Collisions between objects
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are handled by preempting the older object in favor of the new
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one.
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<h2>2. Raw Data Chunk Caching</h2>
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<p>Raw data chunks are cached because I/O requests at the
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application level typically don't map well to chunks at the
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storage level. The chunk cache has a maximum size in bytes
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set with the file access property list (defaults to 1MB) and
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when the limit is reached chunks are preempted based on the
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following set of heuristics.
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<ul>
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<li>Chunks which have not been accessed for a long time
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relative to other chunks are penalized.
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<li>Chunks which have been accessed frequently in the recent
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past are favored.
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<li>Chunks which are completely read and not written, completely
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written but not read, or completely read and completely
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written are penalized according to <em>w0</em>, an
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application-defined weight between 0 and 1 inclusive. A weight
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of zero does not penalize such chunks while a weight of 1
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penalizes those chunks more than all other chunks. The
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default is 0.75.
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<li>Chunks which are larger than the maximum cache size do not
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participate in the cache.
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</ul>
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<p>One should choose large values for <em>w0</em> if I/O requests
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typically do not overlap but smaller values for <em>w0</em> if
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the requests do overlap. For instance, reading an entire 2d
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array by reading from non-overlapping "windows" in a row-major
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order would benefit from a high <em>w0</em> value while reading
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a diagonal accross the dataset where each request overlaps the
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previous request would benefit from a small <em>w0</em>.
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<h2>3. Data Caching Operations</h2>
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<p>The cache parameters for both caches are part of a file access
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property list and are set and queried with this pair of
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functions:
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<dl>
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<dt><code>herr_t H5Pset_cache(hid_t <em>plist</em>, unsigned int
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<em>mdc_nelmts</em>, size_t <em>rdcc_nbytes</em>, double
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<em>w0</em>)</code>
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<dt><code>herr_t H5Pget_cache(hid_t <em>plist</em>, unsigned int
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*<em>mdc_nelmts</em>, size_t *<em>rdcc_nbytes</em>, double
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<em>w0</em>)</code>
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<dd>Sets or queries the meta data cache and raw data chunk cache
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parameters. The <em>plist</em> is a file access property
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list. The number of elements (objects) in the meta data cache
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is <em>mdc_nelmts</em>. The total size of the raw data chunk
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cache and the preemption policy is <em>rdcc_nbytes</em> and
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<em>w0</em>. For <code>H5Pget_cache()</code> any (or all) of
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the pointer arguments may be null pointers.
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</dl>
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<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="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="index.html">Other HDF5 documents and links</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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Caching
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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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<address>
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<a href="mailto:hdfhelp@ncsa.uiuc.edu">HDF Help Desk</a>
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</address>
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<!-- Created: Tue May 26 15:20:14 EDT 1998 -->
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<!-- hhmts start -->
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Last modified: 13 December 1999
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<!-- hhmts end -->
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<br>
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Describes HDF5 Release 1.5, Unreleased Development Branch
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</body>
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</html>
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