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Date:      Thu, 06 Feb 2020 20:59:09 +0000
From:      bugzilla-noreply@freebsd.org
To:        bugs@FreeBSD.org
Subject:   [Bug 243946] mdconfig -d causes iostat glitch
Message-ID:  <bug-243946-227@https.bugs.freebsd.org/bugzilla/>

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D243946

            Bug ID: 243946
           Summary: mdconfig -d causes iostat glitch
           Product: Base System
           Version: 12.1-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: jfc@mit.edu

iostat glitched when I used mdconfig -d:

       tty            nvd0              da0              da1             cpu
 tin  tout  KB/t tps  MB/s   KB/t tps  MB/s   KB/t tps  MB/s  us ni sy in id
   0     0 116.07  99 11.22  95.51 379 35.32  95.36 381 35.47   0  0  5  0 =
95
   2    25 114.79  91 10.23  110.73 395 42.69  111.10 390 42.28   0  0  1  =
0 99
   1     6 117.03 125 14.27   0.00 1841192139503780665 3460.45   0.00
1841192139503796095 14.38   0  0  1  0 99
   2     7 119.17  96 11.21   0.00   0  0.00   0.00   0  0.00   0  0  0  0 =
100

I was copying files off an ISO image using mdconfig to create a special dev=
ice
to mount as a cd9660 filesystem.  The extremely high tps in the above iostat
output happened during the interval when the device was destroyed.

This particular iostat was invoked as "iostat nvd0 da0 da1 10".  da0 and da1
are mirrored drives in a ZFS pool so the true I/O counts should be very
similar.

Based on another iostat invocation without an explicit device list, md0 com=
es
before da0 in whatever order iostat uses.  My guess is destroying md0 chang=
ed
indices into the device statistics table.  Iostat tried to subtract "this
tick's count for index 1 =3D da0" from "last tick's count for index 1 =3D m=
d0" or
something along those lines.

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