title: How does Shared State Cache in Yocto work?
body: <p>Can any one explain to me more comprehensively how the sstate cache works in yocto?</p>
  
  <p><a href="http://www.yoctoproject.org/docs/1.7/bitbake-user-manual/bitbake-user-manual.html#setscene" rel="noreferrer">This explanation</a> is far from clear.</p>
  
  <p>I don't understand when such situation occurs:</p>
  
  <pre><code>NOTE: Preparing runqueue
  NOTE: Executing SetScene Tasks
  NOTE: Running setscene task 118 of 155 (virtual:native:/home/lulianhao/poky-build/edwin/poky/meta/recipes-devtools/pseudo/pseudo_git.bb:do_populate_sysroot_setscene)
  NOTE: Running setscene task 119 of 155 (/home/lulianhao/poky-build/edwin/poky/meta/recipes-devtools/quilt/quilt-native_0.48.bb:do_populate_sysroot_setscene)
  </code></pre>
  
  <p>When it found the artifacts or got the candidates, and then checks the signatures. I want to know when setscene tasks are actually run.</p>
  
  <p>Additional question: When it looks in local sstate_cache folder and when into mirror?</p>