top io mode

Jeremy Chadwick freebsd at
Thu Nov 25 20:57:36 UTC 2010

On Thu, Nov 25, 2010 at 02:28:35PM -0600, Adam Vande More wrote:
> On Thu, Nov 25, 2010 at 2:08 PM, Jeremy Chadwick
> <freebsd at>wrote:
> > >  Well the top bug didn't seem to resolve my actual issue and yes it it a
> > > complete ZFS system.  Problem is that the HD activity indicator light is
> > > constantly flickering even though should be minimal activity.  top still
> > > shows no activity around the blinks, and there's no swapping happening
> > > although gstat does seem to roughly match the blinks.  I can't tell what
> > > your patch does, would it enable me to see what's touching the disk?
> >
> > Please try using gstat(8) instead.
> >
> Like I said gstat shows activity, but I have no way find what process is
> causing the activity unless there is some hidden gstat option I'm unaware
> of.

Sorry, I missed that part of your explanation, my apologies.

> to be more clear, if I have top -m io in one terminal, I see no movement in
> the READ/WRITE columns.
> while polling gstat output I captured an IO "spike"
> the percent busy is at 0.00 for every entry with the exception of these
> spikes.

I don't have an answer for you, other than "maybe top shows a different
'kind' of I/O than what gstat or iostat does".

Only ideas I have:

1) Possibly acct(2) with sa(8) could provide some insight, but I have no
idea how well it works.

2) I see some ps(8) -O parameters labelled "inblk" and "oublk", but I'm
not sure what "block" means given the man page context.  If these are
"disk blocks", then possibly a shell script that repetitively calls ps
with these arguments could narrow it down.

It's too bad, since sar(1M) on Solaris can be used to achieve exactly
what you're looking for -- and the closest thing to that on FreeBSD that
I know of is acct(2) and sa(8).

| Jeremy Chadwick                                   jdc at |
| Parodius Networking              |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |

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