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Kernel Panic - Fallback to Old Kernel

Few days back, i was testing a kernel module, unfortunately it didn't work and instantly after reboot, kernel Panic occurs. Well i was clueless what goes wrong, but now I cannot select the older kernel version to boot. But, there is a way out, if we choose these first before reboot (off course after installing new kernel), and this is inbuilt in GRUB. This mechanism is called Grub Fall-back. In this case you can set your older/tested kernel as the Fallback kernel in case new kernel panics. And here is how you do it:All you have to do is to open your /boot/grub/grub.conf and add a few lines in it like this

—————————————————————————————————————
default=saved

timeout=15
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
fallback 0 1

title CentOS LIDS (2.6.18-128)
root (hd0,0)
kernel /vmlinuz ro root=/dev/VolGroup00/LogVol00
initrd /initrd-lids-2.6.18-128.img
savedefault fallback

title CentOS (2.6.18-128.el5xen)
root (hd0,0)
kernel /vmlinuz-2.6.18-128.el5 ro root=/dev/VolGroup00/LogVol00
initrd /initrd-2.6.18-128.el5.img
savedefault

—————————————————————————————————————

If the above grub.conf is booted, then first Grub.conf will try to boot the system with “CentOS Testing” and if and only if the kenrel panics, it will call “CentOS Working” and boot the system

Hope this quick howto helps.

Citrix XenServer – iSCSI LUN

Hi all, here is my new Mini-Howto for Citrix XenServer – Storage Repository Over Software iSCSI LUN

Topic contains how to and about iSCSI, NFS, on Linux.  How to mount them on Citrix Xen Server using Command line and GUI (that includes Screen Shots)

Let’s starts with iSCSI creation on linux box, I will use CentOS 6 for this.

Packages Required:  scsi-target-utils

Service Name: tgtd

Configuration File: /etc/tgt/targets.conf

I will Use LVM, as i am using  a large partition on this box. So proceeding with it:

# fdisk /dev/sda

Press ‘n’ for new partition

Press ‘L’ for logical partition

First Cylinder size: press enter

Type ‘w’  for save and quit

# init 6  (Restart the System)

Note: do not format partition:

LVM Setup on newly created partition:

Now create Physical Volumes (PV) on newly created partition.

# pvcreate /dev/sda5

# pvscan

# pvdisplay

Now create Volume Group (VG) : on created PV

# vgcreate  sr0_vg1 /dev/sda5

# vgscan

# vgdisplay

Now we can create one or more Logical Volumes (LV’s) on VG

# lvcreate  --size  +30G --name sr0_vg1_lv100 sr0_vg1

# lvscan

# lvdisplay

This part is done: Now to LUN creation

For this we have to ensure we have required package:

# rpm -qa | grep -i scsi

iscsi-initiator-utils-6.2.0.872-34.el6.x86_64

scsi-target-utils-1.0.14-4.el6.x86_64  (this is main package required for iSCSI server)

If do not have, the install it either using directly from RPM if you have, or YUM

# yum install -y scsi-target-utils

After the successful installation, we have to edit the config for SAN:

#  vi /etc/tgt/targets.conf

<target iqn.2012-08.web18:vmsan>

#       Example: the next line would override default driver type.

#       driver iser

backing-store /dev/ sr0_vg1/ name sr0_vg1_lv100

backing-store /dev/ sr0_vg1/ name sr0_vg1_lv101

backing-store /dev/ sr0_vg1/ name sr0_vg1_lv102

#initiator-address 192.168.23.113 (by proving IP, we can restrict the storage to specific IPs only)

#initiator-address 192.168.23.83

</target>

Save and Quit

Now start the service of iSCSI target:

# service tgtd start

#chkconfig tgtd on

# tgt-admin -s     (this will display information about iscsi target device)

I have disabled IPTABLES on target server, in my case.

This done now i will show you how to mount the available LUN on Citrix Xen Server using GUI.

Later I will add command prompt, currently I am facing some issue, suggest me if you have steps

Open XenCenter à Right Click on any XenServer a Choose  New SR....

In Next window, choose Software iSCSI and click Next

Name the New SR and click Next

Provide IP of Target server (iSCSI server) and click on Discover IQNs, choose your IQN and then click on  Discover LUNs and choose your LUN that you had created during LVM create.

Click on Finish and the XenCenter automatically mounts it on Server

And finally here is output/details of mounted SR.

Now LUN is ready and mounted on XenServer, start using as your need.

Please let me know for suggestions/corrections.

Kernel Panic - Fallback to Old Kernel

Few days back, i was testing a kernel module, unfortunately it didn't work and instantly after reboot, kernel Panic occurs. Well i was clueless what goes wrong, but now i can not select the older kernel version to boot. But, there is a way out, if we choose these first before reboot (off course after installing new kernel), and this is inbuilt in GRUB. This mechanism is called Grub Fall-back. In this case you can set your older/tested kernel as the Fallback kernel in case new kernel panics. And here is how you do it:All you have to do is to open your /boot/grub/grub.conf and add a few lines in it like this