Why is the iops observed by fio different from that observed by iostat?
<p>Recently, I'm trying to test my disk using fio. My configuration of fio is as follows:</p>

<pre><code>[global]
invalidate=0    # mandatory
direct=1
#sync=1
fdatasync=1
thread=1
norandommap=1
runtime=10000
time_based=1

[write4k-rand]
stonewall
group_reporting
bs=4k
size=1g
rw=randwrite
numjobs=1
iodepth=1
</code></pre>

<p>In this configuration, you can see that I configured fio to do random writes using direct io. While the test is running, I used iostat to monitor the I/O performance. And I found that: if I set fdatasync to 1, then the iops observed by fio is about 64, while that observed by iostat is about 170. Why is this different? And if I don't configure the "fdatasync", both iops are approximately the same, but much higher, about 450. Why? As far as I know, direct io does not go through page cache, which, in my opinion, means that it should take about the same time not matter whether fdatasync is used.</p>

<p>And I heard that iostat could come up with wrong statistics under some circumstances. Is that real? What exactly circumstance could make iostat go wrong? Is there any other tools that I can use to monitor the I/O performance?</p>