Date: Wed, 05 May 2010 18:03:08 +0300 From: Giorgos Keramidas <keramida@ceid.upatras.gr> To: Peter Steele <psteele@maxiscale.com> Cc: "freebsd-questions@freebsd.org" <freebsd-questions@freebsd.org> Subject: Re: Suspect results from iostat--FBSD bug? Message-ID: <877hnigzmb.fsf@kobe.laptop> In-Reply-To: <7B9397B189EB6E46A5EE7B4C8A4BB7CB3E0CB171@MBX03.exg5.exghost.com> (Peter Steele's message of "Wed, 5 May 2010 07:34:55 -0500") References: <7B9397B189EB6E46A5EE7B4C8A4BB7CB3E0CB171@MBX03.exg5.exghost.com>
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On Wed, 5 May 2010 07:34:55 -0500, Peter Steele <psteele@maxiscale.com> wrote:
> We use iostat to collect statistics of hard drive activity. We've been
> seeing some values for the transaction wait column that look
> suspicious. This is easy to reproduce by just running iostat
> repeatedly over a short period of time, as I show below. Notice the
> third from last column. From what I understand, when run in this
> fashion the values displayed are averaged over the system uptime. I'd
> expect then for the transaction wait value to not take these sudden
> dips. Is there an explanation for this?
>
> # for ((i=1; i <=100; i++)); do iostat -dxI ad8|tail +3; sleep 5; done
> ad8 10291.0 569044.0 151986.0 10164944.0 4294967295 47.7 93
> ad8 10304.0 570070.0 152012.0 10185395.0 4294967295 47.6 93
> ad8 10312.0 571047.0 152028.0 10204575.0 85 47.5 93
This looks like a bug in iostat. 4294967295 == 2 * 32 - -1
It seems that some call returns (unsigned long)-1, e.g. to indicate a
failing system/library call but iostat still prints the result:
: keramida@kobe:/home/keramida$ cat -n demo.c
: 1 #include <limits.h>
: 2 #include <stdio.h>
: 3
: 4 int
: 5 main(void)
: 6 {
: 7 (void)printf("-1 => %lu\n", (unsigned long)-1);
: 8 return 0;
: 9 }
: keramida@kobe:/home/keramida$ cc demo.c
: keramida@kobe:/home/keramida$ ./a.out
: -1 => 4294967295
: keramida@kobe:/home/keramida$
Which _precise_ version of FreeBSD are you using?
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