GPT vs MBR for swap devices

bob prohaska fbsd at
Fri Jun 15 05:15:45 UTC 2018

On Thu, Jun 14, 2018 at 10:00:59PM -0600, Warner Losh wrote:
> I think that's because USB flash can't swap fast enough to keep up with the
> page demand. You might be able to confirm this by looking at the write
> rates to the swap portions for the various other media with gstat. 

I'll try capturing vmstat -c -w 5 to a file, but will need help 
interpreting the results. It seems particularly hard to correlate
vmstat output with corresponding compilation activity. 

> I
> suspect it's FTL is doing more expensive garbage collection under a swap
> work load leading to long pauses from time to time that the VM system
> responds to by starting OOM too soon.

Wouldn't flash speed issues equally affect SD flash and USB flash swap?
>From what I can see SD flash performs far better than USB flash for swap
on the RPI3. Also, using 1 GB USB flash swap together with 1 GB SD flash
swap produced worse performance than the 1 GB SD flash swap alone.

Are there contention issues between USB traffic and SD traffic? I've
always thought they were mostly independent. If they obstruct one 
another that might help explain what I'm seeing. It would also make
clear that my goal of "interleaving" swap devices was badly mistaken. 

It's worth remembering that USB flash swap (2 GB, in a single partiton)
seems to work quite well on an RPI2 running current. If it works on a Pi2
shouldn't it work on a Pi3? 

Thanks for reading!

bob prohaska

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