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.
--
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