zfs.core, 'send' crashing - how to debug offline?

[Harry Schmalzbauer]

Hello,

playing with -current snapshot provides all kinds of surprises :-)

The one  regarding the topic:
'zfs send' aborts with "invalid argument".
The machine in question doesn't have internet access (but local FreeBSD 
FTP mirror, so I managed to install matching base-dbg).
What debugger am I supposed to use these days for -current?  Sorry for 
that ignorant/stupid question – all I know about the transition to 
llvm/clang is that it's still 'cc' ;-)
There's still /usr/libexe/kdgb, but I can't use it for my userland core 
dump, can I?
Not beeing able to dump ZFS is a very severe problem for me and I'd like 
to provide at least some useful problem reports.

Any help highly appreciated, thanks,

-harry

P.S. No matter if I try to send snapshots or unmounted fielsystems - 
'send' is always aborting (scrub doesn't detect inconsistencies).

P.P.S.: sendmail(8) is back in base!?! and DMA gone????????
P.P.P.S.: efi\freebsd\loader.efi doesn't work well – raised and already 
commented by bcran@
P.P.P.P.S: kern.vt.fb.default_mode vs. efi_max_resolution for 
loader.conf(8) was hard to invalidate reading vt(4) – still completely 
unsure how/if to change non-KMS UEFI/GOP resolution outside loader(8).