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<title>WiredTiger: File allocation</title>
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<div class="title">File allocation </div> </div>
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<div class="textblock"><h1><a class="anchor" id="tuning_system_file_block_grow"></a>
File growth</h1>
<p>It's faster on some filesystems to grow a file in chunks rather than to extend it a block at a time as new blocks are written. By configuring the wiredtiger_open functions <code>file_extend</code> value, applications can grow files ahead of the blocks being written.</p>
<div class="fragment"><div class="line"> ret = <a class="code" href="group__wt.html#ga9e6adae3fc6964ef837a62795c7840ed">wiredtiger_open</a>(</div>
<div class="line"> home, NULL, <span class="stringliteral">"create,file_extend=(data=16MB)"</span>, &conn);</div>
</div><!-- fragment --><p> The system calls used to extend files in chunks on some systems require locking across the system call, which may decrease performance. Before configuring a <code>file_extend</code> value, before and after throughput should be carefully evaluated.</p>
<h1><a class="anchor" id="tuning_system_file_block_allocation"></a>
File block allocation</h1>
<p>By default, when file blocks are being reused, WiredTiger attempts to avoid file fragmentation by selecting the smallest available block rather than splitting a larger available block into two. The <code>block_allocation</code> configuration string to <a class="el" href="struct_w_t___s_e_s_s_i_o_n.html#a358ca4141d59c345f401c58501276bbb" title="Create a table, column group, index or file. ">WT_SESSION::create</a> can be set to <code>first</code> to change the algorithm to first-fit, that is, take the first available block in the file. Applications where file size is more of an issue than file fragmentation (for example, applications with fixed-size blocks) might want to configure this way.</p>
<div class="fragment"><div class="line"> ret = session->create(session, <span class="stringliteral">"table:mytable"</span>,</div>
<div class="line"> <span class="stringliteral">"key_format=S,value_format=S,block_allocation=first"</span>);</div>
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