poudriere ports tree create/delete?

[Anton Shterenlikht]

I'm trying to set up poudriere.
I two places I see this command:

http://forums.freebsd.org/showthread.php?t=38859
https://fossil.etoilebsd.net/poudriere/doc/trunk/doc/use_system_ports_tree.wiki

poudriere ports -c -F -f none -M /usr/ports -p default

or

poudriere ports -c -F -f none -M /ports/custom -p custom

So what exactly is the result of this command?
Should something appear under $BASEFS?

My system tree is /usr/ports.
I normally update it via svn+https.
I run the following command:

poudriere ports -c -F -f none -M /usr/ports -p default

and got

# poudriere ports -l
PORTSTREE            METHOD     PATH
default              portsnap   /usr/ports
#

I can't seem to update it:

# poudriere ports -u
====>> Updating portstree "default"
portsnap: Directory does not exist or is not writable: /usr/ports/.snap
# 

I presume it's because the tree is updated
with svn+https and not with portsnap.
So now I want to change the update method to svn+https.
How do I do this?

What happens if I run

poudriere ports -d

Will I delete my /usr/ports?

Thanks

Anton