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    <p><code>iostat -x</code> (I used an old <a href="http://tester-higkoo.googlecode.com/svn-history/r14/trunk/Tools/iostat/iostat.c" rel="noreferrer">source code</a> to write this up before realizing it) displays information from <code>/proc/diskstats</code> (documented <a href="http://www.kernel.org/doc/Documentation/iostats.txt" rel="noreferrer">here</a>) and <code>/proc/stat</code> (for CPU times; see <a href="http://www.kernel.org/doc/man-pages/online/pages/man5/proc.5.html" rel="noreferrer">man proc(5)</a>) (and a few others, but that's not important for understanding).</p> <p>You can see the relevant snippets of code in osgx's answer, but I couldn't make sense of them in isolation, so here's an extended explanation:</p> <ul> <li><code>%util = blkio.ticks / deltams * 100%</code></li> <li><code>deltams</code> is the time elapsed since last snapshot in ms. It uses CPU stats from <code>/proc/stat</code> presumably because it gives better results than to rely on system time, but I don't know for sure. (Side note: for some reason the times are divided by <code>HZ</code>, while the <a href="http://www.kernel.org/doc/man-pages/online/pages/man5/proc.5.html" rel="noreferrer">documentation</a> states it's in <code>USER_HZ</code>, I don't understand that.)</li> <li><p><code>blkio.ticks</code> is "# of milliseconds spent doing I/Os", from <a href="http://www.kernel.org/doc/Documentation/iostats.txt" rel="noreferrer"><code>/proc/diskstats</code> docs</a>:</p> <pre><code>Field 9 -- # of I/Os currently in progress The only field that should go to zero. Incremented as requests are given to appropriate struct request_queue and decremented as they finish. Field 10 -- # of milliseconds spent doing I/Os This field increases so long as field 9 is nonzero. </code></pre> <p>i.e. my understanding is that <code>ticks</code> is the number of ticks when any I/O request (for this device) was in progress multiplied by the duration between ticks.</p></li> </ul> <p>So <code>%util = 100%</code> means that each time the kernel looked (I guess it's 1000 times per second on modern kernels, see "HZ"), an I/O request was in progress.</p> <p>Here's an excerpt from <a href="http://dom.as/2009/03/11/iostat/" rel="noreferrer">another post on iostat</a>:</p> <blockquote> <p>[%util is] how much time did the storage device have outstanding work (was busy).</p> <p>In proper RAID environments it is more like “how much time did at least one disk in RAID array have something to do”. I’m deliberately excluding any kind of cache here – if request can be served from cache, the chance is quite negligible it will show up in %util, unlike in other values.</p> <p>What this also means – the RAID subsystem can be loaded from 6.25% (one disk doing the work) to 100% (all of them busy). Thats quite a lot of insight in single value of ’100%’, isn’t it?</p> </blockquote>
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