Core dump Xen domU/guests

If you’ve been wondering why on_crash='coredump-restart' or on_crash='coredump_restart'(as per some manuals) doesnt work or gives you
“Error: Invalid event handling mode: crash”, here’s why:

I spend hours looking for the answer, and no one one ##xen would also tell me why (asked multiple times, and idled long enough)..and after breaking my head for hours I found the answer.
So listen carefully 🙂

A. Xendump can be configured to capture vmcore dumps of para-virtualized (PV) Xen domU/guests automatically upon a crash. ie., by using on_crash=’coredump-restart’

B. Core dumps from fully-virtualized (FV) Xen domU/guests can only be taken manually by running the `xm dump-core` command.

 

Here’s a detailed how-to on enabling coredump for Xen domU/guest:

 

1. On the dom0/Xenserver, edit /etc/xen/xend_config.sxp and uncomment
# (enable-dump no)
and make it
(enable-dump yes)

2. Restart Xend (depending on whether you use init.d scripts or service utility)
# /etc/init.d/xend restart
or
# service xend restart

3. On the domU/Guest, edit domU.cfg (domU’s configuration file) and add/edit line

#If your domU is fully-virtualized
on_crash='restart'

or

#If your domU is para-virtualized
on_crash='coredump-restart'

4. Start domU

# xm create /path/to/domU.cfg

5. Now, if your domU/guest is para-virtualized, login to the domU and

Check if kernel.panic key is true
# sysctl -A | grep -e kernel | grep -e panic

Set them if they are not
# sysctl -w kernel.panic=1
# sysctl -w kernel.panic_on_oops=1

6. Test it from inside domU/Guest. (Para-Virtualized Guest)
Warning: The next line of code will crash your domU for test.

# echo "c" >/proc/sysrq-trigger

Note: The above command is a way of invoking magic sysrq
a. It will not work if sysrq is disabled on the box
b. NO. I am not gonna talk about magic sysrq’s in this post; or even how to find if its disabled 🙂

7. Test it from dom0/Xenserver. (Fully-Virtualized Guest)
Warning: The next line of code will crash your domU for test.

# xm dump-core -C

8. Above steps 6/7 will dump vmcore to /var/xen/dump (default directory unless you have changed it)
Oh, BTW, make sure you create that directory if it doesn’t exists before taking the dump 😀

Enjoy analyzing the dump 😉