iostat strangeness

mike ledoux mwl+gnhlug at alumni.unh.edu
Tue Jan 15 17:22:52 EST 2008


On Tue, Jan 15, 2008 at 04:57:02PM -0500, Bruce Dawson wrote:
> Its been several years (and major kernel versions) since I've played
> with iostat, but perhaps my statements here will goad someone with more
> recent experience to inject more accurate truths...
> 
>     * iostat used to "not work well" on SMP systems.

That's unfortunate.  Hopefully that's been fixed. 

>     * your "510kB/s average write on dm-5, but only 184.01kB/s average
>       write on the enclosing PV?" observation may be due to "write
>       behinds", caching, and seek/latency optimization.

Caching issues was my first thought, but doesn't apply to the 43+
day average numbers in play here.  I don't think any of those other
optimizations would have such a significant effect over periods this
large, either.  Eventually all of the data written to the LV needs
to be written to the PV, right?

>     * iostat essentially just does arithmetic on the counters kept by
>       the kernel.
>     * For long uptimes, counters can overflow and produce some *really
>       strange* numbers. I would expect Linux to use 64 bit counters in
>       recent kernels though.

I'd hope there would be some trap to reset all of the counters to 0
if one overflows, but that may just be dreaming on my part.  That
may be what is happening, though, as the numbers look OK on 10
second intervals.  I suppose I'll have to schedule a reboot to get
decent numbers. *grumble*

> Don't you just love documentation written by developers (I'm referring
> to the iostat man page)?

I like it, but that's just me.

-- 
mwl+gnhlug at alumni.unh.edu          OpenPGP KeyID 0x57C3430B
Holder of Past Knowledge           CS, O-
"I was walking down the street one day when a beautiful bird caught my eye.
 And I had to climb all the way to the top of a tree to get it back."
           Abhishek Gami



More information about the gnhlug-discuss mailing list