initramfs (RHEL 6)

dracut utility can be used in RHEL 6 to rebuild the initrams image ,It  creates  an initial image used by the kernel for preloading the block device modules (such as IDE, SCSI or RAID) which  are  needed  to access the root filesystem.


1.       It is recommended you make a backup copy of the initrd in case the new version has an unexpected problem:

# cp /boot/initramfs-$(uname -r).img /boot/initramfs-$(uname -r).img.bak


2      Now rebuild the initramfs for the current kernel version:

# dracut -f

Note:

If you are in a kernel version different to the initrd we are building (including if you are in Rescue Mode). We must specify the full kernel version, including architecture:


# dracut -f initramfs-2.6.32-71.el6.x86_64.img 2.6.32-71.el6.x86_64

The only purpose of an initramfs is to mount the root filesystem.

The initramfs is a complete set of directories that you would find on a normal root filesystem.

At boot time, the boot loader loads the kernel and the initramfs image into memory and starts the kernel. 

The kernel checks for the presence of the initramfs and, if found, mounts it as / and runs /init. 

The init program is typically a shell script. Note that the boot process takes longer, possibly significantly longer, if an initramfs is used. 

Kdump Configuration

Kump Introduction

Kdump is the Linux kernel crash-dump mechanism. Oracle recommends that you enable the Kdump feature. In the event of a system crash, Kdump creates a memory image (vmcore) that can help in determining the cause of the crash. Enabling Kdump requires you to reserve a portion of system memory for exclusive use by Kdump. This memory is unavailable for other uses.

Kdump uses kexec to boot into a second kernel whenever the system crashes. kexec is a fast-boot mechanism which allows a Linux kernel to boot from inside the context of a kernel that is already running without passing through the bootloader stage.


When the kdump crash dumping mechanism is enabled, the system is booted from the context of another kernel. This second kernel reserves a small amount of memory and its only purpose is to capture the core dump image in case the system crashes.

Configuring the kdump Service:
There are three common means of configuring the kdump service: at the first boot, using the Kernel Dump Configuration graphical utility, and doing so manually on the command line.
Configuring the kdump at First Boot: The menu which we see during OS Installation.

Using the Kernel Dump Configuration Utility:

Select System → Administration → Kernel crash dumps from the panel, or type system-config-kdump at a shell prompt

Configuring kdump on the Command Line:
To configure the amount of memory to be reserved for the kdump kernel, edit the /boot/grub/grub.conf file and add crash kernel=M or crash kernel=auto

How to go previous date from current date

[root@node1 ~]# date
Sun Jul 13 05:29:50 EDT 2014

[root@node1 ~]# date -d "30 Days Ago"
Fri Jun 13 05:33:45 EDT 2014