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<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta name="GENERATOR" content="Mozilla/4.75 [en] (Windows NT 5.0; U) [Netscape]">
<title>CFD General Notation System</title>
</head>
<body text="#000000" bgcolor="#FFFFFF">
<table BORDER=0 CELLSPACING=5 CELLPADDING=5 WIDTH="100%" >
<tr>
<td WIDTH="180"><a href="http://www.cgns.org/"><img SRC="cgns.gif" ALT="CGNS logo" BORDER=0 height=111 width=180 align=CENTER></a></td>
<td ALIGN=CENTER>
<h1>
<font color="#000000">CFD General Notation System<br>
Switch to HDF5</font></h1>
</td>
</tr>
<tr>
<td COLSPAN="2">
<hr></td>
<td> </td>
</tr>
<tr>
<td VALIGN=TOP WIDTH="180">
<table BORDER CELLSPACING=0 CELLPADDING=5 WIDTH="100%" >
<tr>
<td VALIGN=TOP>
<p><b>CGNS site pages</b>
<br><a href="index.html">CGNS Home</a>
<br><a href="WhatIsCGNS.html">What is CGNS?</a>
<br><a href="starting.html">Getting Started</a>
<br><a href="news.html">Latest News</a>
<br><b><font color=red>Switch to HDF5</font></b>
<br><a href="steering.html">Steering Committee</a>
<br><a href="Implementations.html">Implementations</a>
<br><a href="CGNStalk.html">Discussion Group</a>
<br><a href="download.html">Download</a>
<br><a href="Utilities.html">Contributed Utilities</a>
<br><a href="CGNSFiles.html">Example CGNS Files</a>
<br><a href="Proposals.html">Proposed Extensions</a>
<br><a href="FAQs.html">FAQs</a>
<p><b>Documentation pages</b>
<br><a href="CGNS_docs_current/index.html">Documentation Home</a>
<br><a href="CGNS_docs_current/index.html#Papers">Papers and Slides</a>
<br><a href="CGNS_docs_current/index.html#Minutes">Meeting Minutes</a>
<br><a href="CGNS_docs_current/user/index.html">User's Guide</a>
<br><a href="CGNS_docs_current/overview/index.html">Overview</a>
<br><a href="CGNS_docs_current/sids/index.html">SIDS</a>
<br><a href="CGNS_docs_current/filemap/index.html">SIDS File Mapping</a>
<br><a href="CGNS_docs_current/midlevel/index.html">Mid-Level Library</a>
<br><a href="CGNS_docs_current/cgio/index.html">CGIO User's Guide</a>
</td>
</tr>
</table>
</td>
<td VALIGN=TOP>The CGNS implementation of SIDS, so-called MLL, was originally
built using a file format called ADF (Advanced Data Format). This
format was based on a common file format system previously in use at McDonnell
Douglas. The ADF has worked extremely well, requiring little repair,
upgrade, or maintenance over the last decade.
<p>However, ADF does not have parallel I/O or data compression capabilities,
and does not have the support and tools that the storage format HDF5
offers. HDF5, supported by
<a href="http://www.hdfgroup.org/HDF5/">The HDF Group</a>,
has rapidly grown to become a world-wide format standard
for storing scientific data. HDF5 has parallel capability as well
as a broader support base than ADF. (The HDF Group is a spin-off from NCSA, as
described on the HDF Group website.)
<p>Therefore, the CGNS Steering Committee has made the decision to adopt
HDF5 as the default (official) data storage mechanism.
However, because it is possible to easily support both ADF
and HDF5 formats simultaneously (giving the user the capability to choose
between them), we will continue to support both indefinitely.
<p>If the user links to HDF5 (version 1.8 or later) when compiling CGNS Version 3.1 or later,
then by <i>default</i> the software will write HDF5 files (although the user can
force the writing of ADF files instead if desired). The software can read either
format without the user having to do anything.
<p>To summarize,
the following is our goal: The CGNS Steering Committee considers HDF5 to
be its official, recommended storage format. However, ADF will continue
to be available for use, with the CGNS mid-level library capable of (1)
using either format and (2) translating back and forth between the two.
<br>
<br>
<br>
<br>
<h5><p><font color=green>NOTE: In the initial implementation of HDF5 in CGNS (CGNS Version 2.5), the index ordering for HDF5
was done incorrectly. CGNS uses the Fortran-convention for index ordering (1st index
varies fastest), whereas the de-facto HDF5 standard is the C-indexing convention.
As a result of this implementation error, partial reads and writes only worked when ADF was being used
as the underlying database manager. They did not work correctly with HDF5.
Furthermore, HDF5 multi-index arrays created with CGNS would not be read correctly by non-CGNS software.
Other than this, users would not have noticed any problems with the HDF5 implementation because the CGNS software was
internally consistent.
Beginning with CGNS Version 2.5.4 software, this bug has been corrected. Also, Version 2.5.4 and later
software can read old CGNS HDF5 files, and automatically detect and correct the
index inversion (transparent to the user).</font></p></h5>
<h5><p><font color=green>NOTE: if building CGNS with an HDF5 version <u>earlier</u> than 1.8, then links will not work
when using HDF5 file type.</font></p></h5> </td>
</tr>
</table>
</body>
</html>