To get dump, you need to follow given below steps
1. Install kexec-tools and crash
yum install kexec-tools crash
2. Enable crashkernel option in grub.conf
kernel /vmlinuz….. crashkernel=128M@16M
This crashkernel parameter reserve memory for dump that is 128M while keeping initial memory allocation to 16M .
3. Reboot the system with crashkernel enabled and verify from command , if crashkernel activated successfully
cat /proc/iomem|grep Crash
4. Start kdump service . Also before starting kdump service you can have a look in /etc/kdump.conf . In /etc/kdump.conf file we can specify location where dump file get stored . Normally dump file get stored inside /var/crash folder.
5. Send “c” to sysrq (http://en.wikipedia.org/wiki/System_request) using following command