GPT vs MBR for swap devices

[bob prohaska]

On Fri, Jun 15, 2018 at 11:21:01AM -0600, Warner Losh wrote:
> On Fri, Jun 15, 2018 at 10:42 AM, bob prohaska <fbsd at www.zefox.net> wrote:
> 
> > On Fri, Jun 15, 2018 at 09:57:17AM -0600, Warner Losh wrote:
> >
> > > gstat -Bd -I 10 -f <regexp for your swap partition>  >
> > gstat-swap-data.dat
> > >
Could a longer interval, maybe one or ten seconds, and perhaps the -a
flag to reduce file size produce useful results? Given the way gstat 
output is formatted it does not look easy to search, except in small 
amounts by eye. 

> > > would be how I'd recommend collecting it. This file may get kinda big
> > > depending how long it takes to trigger the weird state. I'm hoping that
> > if
> > > you put this on a known good device, we'll power through the issues. We
> > > might not get perfect correlation with this, but the data should show all
> > > kinds of crazy before the system drives off the cliff if I'm right, so we
> > > don't need perfect data.
> > >
> >
> > Apologies,  we spoke of gstat earlier. I lost the distinction between
> > gstat and vmstat. In the case at hand the possible swap devices are
> >
> > /dev/da0b (this is the one causing trouble)
> > /dev/mmcsd0s3b
> > /dev/mmcsd0s3d
> >
> >
> > what would the correct <regexp> look like?
> >
> 
> I'd just use -f '^da0b$' for each of the devices.
> 
So, ignore the SD flash for now?

> 
> > Is there any indication why problems seen on the RPI3 do not seem
> > apparent on an RPI2? A 2 GB USB flash swap partition seems to work
> > successfully on the Pi2.
> >
> 
> Interesting. The only thing I can think of is different OOM tuning levels.
> But if anything, I'd expect more trouble on the RPi2 than the RPi3. So that
> might also point to a difference in USB and/or VM things. It would be
> interesting to see if we see a significant difference in the graphs between
> the two for the same physical drive. I'd have expected, like you, that this
> would be reproducible between the two.
>

The Pi2 vs Pi3 comparison is less fair than I indicated; the Pi2 is running
11.2-PRERELEASE FreeBSD 11.2-PRERELEASE #0 r335072, while the Pi3 is running
12.0-CURRENT FreeBSD 12.0-CURRENT #0 r334939. Both Pi2 and Pi3 are using 64
GB Sandisk Extreme USB flash drives, but the unit on the Pi3 is about a year
newer. I'd expect that to help, rather than hurt, but.... 

I'll try logging gstat output on the Pi2 as-is during a clean-start 
buildworld and see what happens.

Thanks for all your help!

bob prohaska