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<!-- #EndLibraryItem --><h1>Data Caching</h1>

    <h2>1. Meta Data Caching</h2>

    <p>The HDF5 library caches two types of data: meta data and raw
      data.  The meta data cache holds file objects like the file
      header, symbol table nodes, global heap collections, object
      headers and their messages, etc. in a partially decoded
      state. The cache has a fixed number of entries which is set with 
      the file access property list (defaults to 10k) and each entry
      can hold a single meta data object.  Collisions between objects
      are handled by preempting the older object in favor of the new
      one.

    <h2>2. Raw Data Chunk Caching</h2>

    <p>Raw data chunks are cached because I/O requests at the
      application level typically don't map well to chunks at the
      storage level.  The chunk cache has a maximum size in bytes
      set with the file access property list (defaults to 1MB) and
      when the limit is reached chunks are preempted based on the
      following set of heuristics.

    <ul>
      <li>Chunks which have not been accessed for a long time
	relative to other chunks are penalized.
      <li>Chunks which have been accessed frequently in the recent
	past are favored.
      <li>Chunks which are completely read and not written, completely 
	written but not read, or completely read and completely
	written are penalized according to <em>w0</em>, an
	application-defined weight between 0 and 1 inclusive. A weight 
	of zero does not penalize such chunks while a weight of 1
	penalizes those chunks more than all other chunks.  The
	default is 0.75.
      <li>Chunks which are larger than the maximum cache size do not
	participate in the cache.
    </ul>

    <p>One should choose large values for <em>w0</em> if I/O requests
      typically do not overlap but smaller values for <em>w0</em> if
      the requests do overlap.  For instance, reading an entire 2d
      array by reading from non-overlapping "windows" in a row-major
      order would benefit from a high <em>w0</em> value while reading
      a diagonal accross the dataset where each request overlaps the
      previous request would benefit from a small <em>w0</em>.

    <h2>3. Data Caching Operations</h2>

    <p>The cache parameters for both caches are part of a file access
      property list and are set and queried with this pair of
      functions:

    <dl>
      <dt><code>herr_t H5Pset_cache(hid_t <em>plist</em>, unsigned int 
	  <em>mdc_nelmts</em>, size_t <em>rdcc_nbytes</em>, double
	  <em>w0</em>)</code>
      <dt><code>herr_t H5Pget_cache(hid_t <em>plist</em>, unsigned int 
	  *<em>mdc_nelmts</em>, size_t *<em>rdcc_nbytes</em>, double
	  <em>w0</em>)</code>
      <dd>Sets or queries the meta data cache and raw data chunk cache 
	parameters.  The <em>plist</em> is a file access property
	list.  The number of elements (objects) in the meta data cache 
	is <em>mdc_nelmts</em>.  The total size of the raw data chunk
	cache and the preemption policy is <em>rdcc_nbytes</em> and
	<em>w0</em>.  For <code>H5Pget_cache()</code> any (or all) of
	the pointer arguments may be null pointers.
    </dl>


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Describes HDF5 Release 1.4.5, February 2003
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