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<div class="title">Performance monitoring with statistics </div> </div>
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<div class="textblock"><p>WiredTiger optionally maintains a variety of statistics, when the <code>statistics</code> configuration string is specified to <a class="el" href="group__wt.html#gacbe8d118f978f5bfc8ccb4c77c9e8813" title="Open a connection to a database. ">wiredtiger_open</a>; see <a class="el" href="statistics.html">Statistics</a> for general information about statistics, and <a class="el" href="data_sources.html#data_statistics">Statistics Data</a> for information about accessing the statistics.</p>
<p><b> Note that maintaining run-time statistics involves updating shared-memory data structures and may decrease application performance. </b></p>
<p>The statistics gathered by WiredTiger can be combined to derive information about the system's behavior. For example, a cursor can be opened on the statistics for a table:</p>
<div class="fragment"><div class="line"> error_check(session->open_cursor(session, <span class="stringliteral">"statistics:table:access"</span>, NULL, NULL, &cursor));</div></div><!-- fragment --><p> Then this code calculates the "fragmentation" of a table, defined here as the percentage of the table that is not part of the current checkpoint:</p>
<div class="fragment"><div class="line"> int64_t ckpt_size, file_size, percent;</div><div class="line"> get_stat(cursor, <a class="code" href="group__wt.html#ga3ba4d6c12abe10285dc3b599f082a4e4">WT_STAT_DSRC_BLOCK_CHECKPOINT_SIZE</a>, &ckpt_size);</div><div class="line"> get_stat(cursor, <a class="code" href="group__wt.html#ga2af5c768712ce6c6b840badd17dd13b9">WT_STAT_DSRC_BLOCK_SIZE</a>, &file_size);</div><div class="line"></div><div class="line"> percent = 0;</div><div class="line"> <span class="keywordflow">if</span> (file_size != 0)</div><div class="line"> percent = 100 * ((file_size - ckpt_size) / file_size);</div><div class="line"> printf(<span class="stringliteral">"Table is %"</span> PRId64 <span class="stringliteral">"%% fragmented\n"</span>, percent);</div></div><!-- fragment --><p> The following example calculates the "write amplification", defined here as the ratio of bytes written to the filesystem versus the total bytes inserted, updated and removed by the application.</p>
<div class="fragment"><div class="line"> int64_t app_insert, app_remove, app_update, fs_writes;</div><div class="line"></div><div class="line"> get_stat(cursor, <a class="code" href="group__wt.html#gaec7145b45392656825b8404ae1fb4f46">WT_STAT_DSRC_CURSOR_INSERT_BYTES</a>, &app_insert);</div><div class="line"> get_stat(cursor, <a class="code" href="group__wt.html#gaf9cc721c75f42a110e0f6af104087b4d">WT_STAT_DSRC_CURSOR_REMOVE_BYTES</a>, &app_remove);</div><div class="line"> get_stat(cursor, <a class="code" href="group__wt.html#ga21596e9d2f3d0c992bcade65b85cb5eb">WT_STAT_DSRC_CURSOR_UPDATE_BYTES</a>, &app_update);</div><div class="line"></div><div class="line"> get_stat(cursor, <a class="code" href="group__wt.html#ga212a7e9d219e279da8b10adfaaab2afc">WT_STAT_DSRC_CACHE_BYTES_WRITE</a>, &fs_writes);</div><div class="line"></div><div class="line"> <span class="keywordflow">if</span> (app_insert + app_remove + app_update != 0)</div><div class="line"> printf(<span class="stringliteral">"Write amplification is %.2lf\n"</span>,</div><div class="line"> (<span class="keywordtype">double</span>)fs_writes / (app_insert + app_remove + app_update));</div></div><!-- fragment --><p> Both examples use this helper function to retrieve statistics values from a cursor:</p>
<div class="fragment"><div class="line"><span class="keywordtype">void</span></div><div class="line">get_stat(<a class="code" href="struct_w_t___c_u_r_s_o_r.html">WT_CURSOR</a> *cursor, <span class="keywordtype">int</span> stat_field, int64_t *valuep)</div><div class="line">{</div><div class="line"> <span class="keyword">const</span> <span class="keywordtype">char</span> *desc, *pvalue;</div><div class="line"></div><div class="line"> cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#ad1088d719df40babc1f57d086691ebdc">set_key</a>(cursor, stat_field);</div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#a7e25b2ced2cf3ec68bd5429bf921c79f">search</a>(cursor));</div><div class="line"> error_check(cursor-><a class="code" href="struct_w_t___c_u_r_s_o_r.html#af85364a5af50b95bbc46c82e72f75c01">get_value</a>(cursor, &desc, &pvalue, valuep));</div><div class="line">}</div></div><!-- fragment --></div></div><!-- contents -->
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