188 lines
6.7 KiB
HTML
188 lines
6.7 KiB
HTML
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<!DOCTYPE html>
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<html><head>
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<meta name="viewport" content="width=device-width, initial-scale=1.0">
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<meta http-equiv="content-type" content="text/html; charset=UTF-8">
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<link href="sqlite.css" rel="stylesheet">
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<title>Internal Versus External BLOBs</title>
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<!-- path= -->
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</head>
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<body>
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<div class=nosearch>
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<a href="index.html">
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<img class="logo" src="images/sqlite370_banner.gif" alt="SQLite" border="0">
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</a>
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<div><!-- IE hack to prevent disappearing logo --></div>
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<div class="tagline desktoponly">
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Small. Fast. Reliable.<br>Choose any three.
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</div>
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<div class="menu mainmenu">
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<ul>
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<li><a href="index.html">Home</a>
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<li class='mobileonly'><a href="javascript:void(0)" onclick='toggle_div("submenu")'>Menu</a>
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<li class='wideonly'><a href='about.html'>About</a>
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<li class='desktoponly'><a href="docs.html">Documentation</a>
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<li class='desktoponly'><a href="download.html">Download</a>
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<li class='desktoponly'><a href="prosupport.html">Purchase</a>
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<li class='search' id='search_menubutton'>
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<a href="javascript:void(0)" onclick='toggle_search()'>Search</a>
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</ul>
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</div>
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<div class="menu submenu" id="submenu">
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<ul>
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<li><a href='about.html'>About</a>
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<li><a href='docs.html'>Documentation</a>
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<li><a href='download.html'>Download</a>
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<li><a href='support.html'>Support</a>
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<li><a href='prosupport.html'>Purchase</a>
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</ul>
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</div>
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<div class="searchmenu" id="searchmenu">
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<form method="GET" action="search">
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<select name="s" id="searchtype">
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<option value="d">Search Documentation</option>
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<option value="c">Search Changelog</option>
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</select>
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<input type="text" name="q" id="searchbox" value="">
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<input type="submit" value="Go">
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</form>
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</div>
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</div>
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<script>
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function toggle_div(nm) {
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var w = document.getElementById(nm);
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if( w.style.display=="block" ){
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}else{
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w.style.display = "block";
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}
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}
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function toggle_search() {
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var w = document.getElementById("searchmenu");
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if( w.style.display=="block" ){
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w.style.display = "none";
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} else {
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w.style.display = "block";
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setTimeout(function(){
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document.getElementById("searchbox").focus()
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}, 30);
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}
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}
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function div_off(nm){document.getElementById(nm).style.display="none";}
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window.onbeforeunload = function(e){div_off("submenu");}
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/* Disable the Search feature if we are not operating from CGI, since */
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/* Search is accomplished using CGI and will not work without it. */
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if( !location.origin || !location.origin.match || !location.origin.match(/http/) ){
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document.getElementById("search_menubutton").style.display = "none";
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}
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/* Used by the Hide/Show button beside syntax diagrams, to toggle the */
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function hideorshow(btn,obj){
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var x = document.getElementById(obj);
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var b = document.getElementById(btn);
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if( x.style.display!='none' ){
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x.style.display = 'none';
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b.innerHTML='show';
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}else{
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x.style.display = '';
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b.innerHTML='hide';
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}
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return false;
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}
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</script>
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</div>
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<body>
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<h1 align="center">
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Internal Versus External BLOBs in SQLite
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</h1>
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<p>
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If you have a database of large BLOBs, do you get better read performance
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when you store the complete
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BLOB content directly in the database or is it faster to store each BLOB
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in a separate file and store just the corresponding filename in the database?
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</p>
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<p>
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To try to answer this, we ran 49 test cases with various BLOB sizes and
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SQLite page sizes on a Linux workstation (Ubuntu circa 2011 with the
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Ext4 filesystem on a fast SATA disk).
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For each test case, a database was created that contains 100MB of BLOB
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content. The sizes of the BLOBs ranged from 10KB to 1MB. The number
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of BLOBs varied in order to keep the total BLOB content at about 100MB.
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(Hence, 100 BLOBs for the 1MB size and 10000 BLOBs for the 10K size and
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so forth.) SQLite <a href="releaselog/3_7_8.html">version 3.7.8</a> (2011-09-19) was used.
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</p>
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<blockquote><i>
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Update: New measurements for SQLite version 3.19.0
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(2017-05-22) show that SQLite is about
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<a href="fasterthanfs.html">35% faster</a> than direct disk I/O for
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both reads and writes of 10KB blobs.
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</i></blockquote>
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<p>
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The matrix below shows the time needed to read BLOBs stored in separate files
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divided by the time needed to read BLOBs stored entirely in the database.
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Hence, for numbers larger than 1.0, it is faster to store the BLOBs directly
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in the database. For numbers smaller than 1.0, it is faster to store the BLOBs
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in separate files.
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</p>
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<p>
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In every case, the pager cache size was adjusted to keep the amount of
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cache memory at about 2MB.
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For example, a 2000 page cache was used for 1024 byte pages
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and a 31 page cache was used for 65536 byte pages.
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The BLOB values were read in a random order.
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</p>
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<table border="1" cellpadding="5" cellspacing="0" align="center">
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<tr>
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<th rowspan="2">Database Page Size<th colspan="7">BLOB size
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<tr>
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<th>10k<th>20k<th>50k<th>100k<th>200k<th>500k<th>1m
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<tr>
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<td>1024<td bgcolor="#b1d3a8">1.535<td bgcolor="#b1d3a8">1.020<td bgcolor="#ffc0d0">0.608<td bgcolor="#ffc0d0">0.456<td bgcolor="#ffc0d0">0.330<td bgcolor="#ffc0d0">0.247<td bgcolor="#ffc0d0">0.233
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<tr>
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<td>2048<td bgcolor="#b1d3a8">2.004<td bgcolor="#b1d3a8">1.437<td bgcolor="#ffc0d0">0.870<td bgcolor="#ffc0d0">0.636<td bgcolor="#ffc0d0">0.483<td bgcolor="#ffc0d0">0.372<td bgcolor="#ffc0d0">0.340
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<tr>
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<td>4096<td bgcolor="#b1d3a8">2.261<td bgcolor="#b1d3a8">1.886<td bgcolor="#b1d3a8">1.173<td bgcolor="#ffc0d0">0.890<td bgcolor="#ffc0d0">0.701<td bgcolor="#ffc0d0">0.526<td bgcolor="#ffc0d0">0.487
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<tr>
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<td>8192<td bgcolor="#b1d3a8">2.240<td bgcolor="#b1d3a8">1.866<td bgcolor="#b1d3a8">1.334<td bgcolor="#b1d3a8">1.035<td bgcolor="#ffc0d0">0.830<td bgcolor="#ffc0d0">0.625<td bgcolor="#ffc0d0">0.720
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<tr>
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<td>16384<td bgcolor="#b1d3a8">2.439<td bgcolor="#b1d3a8">1.757<td bgcolor="#b1d3a8">1.292<td bgcolor="#b1d3a8">1.023<td bgcolor="#ffc0d0">0.829<td bgcolor="#ffc0d0">0.820<td bgcolor="#ffc0d0">0.598
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<tr>
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<td>32768<td bgcolor="#b1d3a8">1.878<td bgcolor="#b1d3a8">1.843<td bgcolor="#b1d3a8">1.296<td bgcolor="#ffc0d0">0.981<td bgcolor="#ffc0d0">0.976<td bgcolor="#ffc0d0">0.675<td bgcolor="#ffc0d0">0.613
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<tr>
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<td>65536<td bgcolor="#b1d3a8">1.256<td bgcolor="#b1d3a8">1.255<td bgcolor="#b1d3a8">1.339<td bgcolor="#ffc0d0">0.983<td bgcolor="#ffc0d0">0.769<td bgcolor="#ffc0d0">0.687<td bgcolor="#ffc0d0">0.609
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</table>
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<p>We deduce the following rules of thumb from the matrix above:</p>
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<ul>
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<li><p>
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A database page size of 8192 or 16384 gives the best performance
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for large BLOB I/O.
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<li><p>For BLOBs smaller than 100KB, reads are faster when
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the BLOBs are stored directly in the database file. For BLOBs larger than
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100KB, reads from a separate file are faster.
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</ul>
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<p>Of course, your mileage may vary depending on hardware, filesystem,
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and operating system. Double-check these figures on target hardware
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before committing to a particular design.
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