docs/gsg_txn/CXX/reversesplit.html

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   6     <title>Reverse BTree Splits</title>
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  10     <link rel="up" href="txnconcurrency.html" title="Chapter 4. Concurrency" />
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  17         <p>Library Version 11.2.5.3</p>
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  19       <table width="100%" summary="Navigation header">
  20         <tr>
  21           <th colspan="3" align="center">Reverse BTree Splits</th>
  22         </tr>
  23         <tr>
  24           <td width="20%" align="left"><a accesskey="p" href="txnnowait.html">Prev</a> </td>
  25           <th width="60%" align="center">Chapter 4. Concurrency</th>
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  31     <div class="sect1" lang="en" xml:lang="en">
  32       <div class="titlepage">
  33         <div>
  34           <div>
  35             <h2 class="title" style="clear: both"><a id="reversesplit"></a>Reverse BTree Splits</h2>
  36           </div>
  37         </div>
  38       </div>
  39       <p>
  40             If your application is using the Btree access method, and your
  41             application is repeatedly deleting then adding records to your
  42             database, then you might be able to reduce lock contention by
  43             turning off reverse Btree splits.
  44         </p>
  45       <p>
  46             As pages are emptied in a database, DB attempts to
  47             delete empty pages in order to keep
  48             the database as small as possible and minimize search time.
  49             Moreover, when a page in the database fills
  50             up, DB, of course, adds additional pages
  51             to make room for more data.
  52         </p>
  53       <p>
  54             Adding and deleting pages in the database requires that the
  55             writing thread lock the parent page. Consequently, as the
  56             number of pages in your database diminishes, your application
  57             will see increasingly more lock contention; the maximum level
  58             of concurrency in a database of two pages is far smaller than
  59             that in a database of 100 pages, because there are fewer pages
  60             that can be locked.
  61         </p>
  62       <p>
  63             Therefore, if you prevent the database from being reduced to a
  64             minimum number of pages, you can improve your application's
  65             concurrency throughput. Note, however, that you should do so
  66             only if your application tends to delete and then add the same
  67             data. If this is not the case, then preventing reverse Btree
  68             splits can harm your database search time.
  69         </p>
  70       <p>
  71             To turn off reverse Btree splits,
  72                 <span>
  73                     provide the
  74                     <code class="literal">DB_REVSPLITOFF</code> flag to the
  75
  76                         <code class="methodname">Db::set_flags()</code>
  77                     method.
  78                 </span>
  79
  80         </p>
  81       <p>
  82             For example:
  83         </p>
  84       <pre class="programlisting">#include "db_cxx.h"
  85
  86 ...
  87
  88 int main(void)
  89 {
  90     u_int32_t env_flags = DB_CREATE     |  // If the environment does not
  91                                            // exist, create it.
  92                           DB_INIT_LOCK  |  // Initialize locking
  93                           DB_INIT_LOG   |  // Initialize locking
  94                           DB_INIT_MPOOL |  // Initialize the cache
  95                           DB_THREAD     |  // Free-thread the env handle
  96                           DB_INIT_TXN;     // Initialize transactions
  97
  98     u_int32_t db_flags = DB_CREATE | DB_AUTO_COMMIT;
  99     Db *dbp = NULL;
 100     const char *file_name = "mydb.db";
 101
 102     std::string envHome("/export1/testEnv");
 103     DbEnv myEnv(0);
 104
 105     try {
 106
 107         myEnv.open(envHome.c_str(), env_flags, 0);
 108         dbp = new Db(&amp;myEnv, 0);
 109
 110         // Turn off BTree reverse split.
 111         dbp=&gt;set_flags(DB_REVSPLITOFF);
 112
 113         dbp-&gt;open(dbp,        // Pointer to the database
 114                   NULL,       // Txn pointer
 115                   file_name,  // File name
 116                   NULL,       // Logical db name
 117                   DB_BTREE,   // Database type (using btree)
 118                   db_flags,   // Open flags
 119                   0);         // File mode. Using defaults
 120
 121     } catch(DbException &amp;e) {
 122         std::cerr &lt;&lt; "Error opening database and environment: "
 123                   &lt;&lt; file_name &lt;&lt; ", " &lt;&lt; envHome &lt;&lt; std::endl;
 124         std::cerr &lt;&lt; e.what() &lt;&lt; std::endl;
 125     }
 126
 127     try {
 128         dbp-&gt;close(dbp, 0);
 129         myEnv.close(0);
 130     } catch(DbException &amp;e) {
 131         std::cerr &lt;&lt; "Error closing database and environment: "
 132                   &lt;&lt; file_name &lt;&lt; ", " &lt;&lt; envHome &lt;&lt; std::endl;
 133         std::cerr &lt;&lt; e.what() &lt;&lt; std::endl;
 134         return (EXIT_FAILURE);
 135     }
 136
 137     return (EXIT_SUCCESS);
 138 } </pre>
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