dracut-initqueue timeout - starting timeout scripts messages while booting in RHEL operating system

When we perform operating system patching on RHEL systems, we will get below error during boot time.

Kernel Panic Error - not syncing: VFS:Unable to mount root fs on unknown-block(0,0)

Kernel Panic crash your system if it is occur. In Unix system, normally such kernel panic occur while we are upgrading the kernel or firmware of any Unix operating system.

In simple words, its look like a blue screen when occur on your personal laptop etc. but if we talk about Unix, the below error message appear during boot time.

Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
Pid: 1, comm: swapper Not tainted 2.6.32-502.el6.x86_64 #1
Call Trace:
[<ffffffff815292bc>] ? panic+0xa7/0x16f
[<ffffffff81c2b432>] ? mount_block_root+0x216/0x2cb
[<ffffffff81002930>] ? bstat+0x2b0/0x980
[<ffffffff81c2b53d>] ? mount_root+0x56/0x5a
[<ffffffff81c2b6b1>] ? prepare_namespace+0x170/0x1a9
[<ffffffff81c2a92a>] ? kernel_init+0x2e1/0x2f7
[<ffffffff8100c20a>] ? child_rip+0xa/0x20
[<ffffffff81c2a649>] ? kernel_init+0x0/0x2f7
[<ffffffff8100c200>] ? child_rip+0x0/0x20

when such above error occur that means your system is in dangerous state and if you not rectify this issue you may lost your data as well. 

We will provide to you one of cause and solution of this error, why it is occur and how we can resolve it.

In one of case, above error message occur when you upgrade your kernel and by mistake if it is not properly upgrade then kernel version get different with previous one.

For resolving this issue you need to boot you unix machine in grub mode. You can take a reboot of server and press any key once the boot screen appear. In the below screen it is showing different option, please select the grub menu option.

In grub mode, check the kernel and initrd parameter, if both parameter kernel version and initrd version are not matched that's means this is the root cause. So you need to correct the kernel version and make a same version entry and boot the operating system after saving the grub entry for temporary purpose.

To fix the permanent this issue, you need to change the kernel version in grub.conf file otherwise once you take a reboot of server again, you have to face same issue. So avoiding such issue again, please correct the version in grub configuration file.

[root@localhost]# vi /etc/grub.conf

#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title CentOS 6 (2.6.32-504.el6.x86_64)
        root (hd0,0)
        kernel /vmlinuz-2.6.32-504.el6.x86_64 ro root=/dev/mapper/vstorage-root rd_NO_LUKS LANG=en_US.UTF-8 rd_NO_MD SYSFONT=latarcyrheb-sun16 crashkernel=auto rd_LVM_LV=vstorage/root  KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM rhgb quiet
        initrd /initramfs-2.6.32-504.el6.x86_64.img

Please do same version entry on bold highlighted place. The kernel version should be same at kernel and initrd line. After changing the entry save the entry and take a reboot of server again to check the kernel panic problem again occur or not.