Red Hat Enterprise Linux Kernel Upgrade

Kernel Upgrade Introduction

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In this KB, we will see how to upgrade the kernel on Red Hat Linux Enterprise systems. We will show you how to perform a automated upgrades of the kernel using the yum and rpm package management tools.

Before continuing on, a word of caution. I recommend avoiding kernel updates unless absolutely necessary. Generally speaking, if it's not broke, don't try and fix it. So unless there is a vulnerability or lack of hardware/driver support in the currently installed kernel on your system, I would recommend against the installation of a new/updated kernel. Installing a new kernel could have wide spread and unknown consequences to your system and/or might not be supported by your hardware/installed software. Now that I got that off my chest, I'll get off my soapbox so we can get to work!

Kernel Upgrade/Update on Red Hat Enterprise Linux

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Red Hat use a combination of tools such as yum for package retrieval and rpm for package management. If you system is registered with the Red Hat Network (RHN), yum will automatically download and resolve dependencies for rpm based installs. If you system is not registered with RHN, you can still upgrade the kernel, but you will have to manually resolve any dependencies (downloading and installing any dependent rpms.) Yum will connect to the RHN (Red Hat Network) and scan the repository for updated packages (including your kernel.) If there are updates available, yum will resolve their dependencies and mark the additional packages for download/installation. Once yum has downloaded all the package(s), it will begin installing the packages using rpm (this will configure and register the package with the OS.)

1. Before we can use yum, we must first register the server with the RHN (Red Hat Network).

Note: If you registered your OS at the time of installation, you can skip this step

Begin the registration process

[root@RHEL01 ~]# rhn_register

You'll be presented with a screen similar to the following, click Next to continue:

Kernel Upgrade Introduction

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Kernel Upgrade/Update on Red Hat Enterprise Linux

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1. Before we can use yum, we must first register the server with the RHN (Red Hat Network).

Note: If you registered your OS at the time of installation, you can skip this step

Begin the registration process

[root@RHEL01 ~]# rhn_register

You'll be presented with a screen similar to the following, click Next to continue:

Enter your login information for the Red Hat Network (RHN) and click Next:

Enter a profile name for the server and click Next:

Leave the defaults selected and click Next:

Click Next on the Send Profile page:

Your system's information will be sent/registered with the Red Hat Network (RHN):

Click OK on the subscription details page:

Click Finish to complete the registration with the RHN:

3. Check to see if there are updates available for the system

Check for updates to the kernel package only:

[root@RHEL01 ~]# yum check-update kernel

Loaded plugins: rhnplugin

kernel.x86_64 2.6.18-128.1.14.el5 rhel-x86_64-server-5

root@ubuntu:~#

4. Note: If there are no updates available, no output will be produced

Check for all packages (including the kernel):

[root@RHEL01 ~]# yum check-update

Loading "installonlyn" plugin

Loading "rhnplugin" plugin

Setting up repositories

rhel-x86_64-server-5 100% |=========================| 1.3 kB 00:00

Reading repository metadata in from local files

primary.xml.gz 100% |=========================| 2.3 MB 00:01

################################################## 7019/7019

Deployment_Guide-en-US.noarch 5.2-11 rhel-x86_64-serv

NetworkManager.x86_64 1:0.7.0-4.el5_3 rhel-x86_64-serv

ORBit2.x86_64 2.14.3-5.el5 rhel-x86_64-serv

OUTPUT TRUNCATED

kernel.x86_64 2.6.18-128.1.14.el5 rhel-x86_64-serv

kernel-headers.x86_64 2.6.18-128.1.14.el5 rhel-x86_64-serv

OUTPUT TRUNCATED

[root@Linux01 rhn]#

5. Note: yum returns a list of all available packages

6. A quick check of the system reveals we are running on v2.6.18-8.el5 of the Linux kernel

[root@RHEL01 ~]# rpm -qi kernel

Name : kernel Relocations : (not relocatable)

Version : 2.6.18 Vendor : Red Hat, Inc.

Release : 8.el5 Build Date : Fri 26 Jan 2007 02:47:08 PM EST

Install Date : Wed 24 Jun 2009 8:50:56 AM EDT Build Host : ls20-bc1-14.build.redhat.com

Group : System Environment/Kernel Source RPM : kernel-2.6.18-8.el5.src.rpm

Size : 75875879 License : GPLv2

Signature : DSA/SHA1, Fri 26 Jan 2007 09:12:24 PM EST, Key ID 5326810137017186

Packager : Red Hat, Inc. <http://bugzilla.redhat.com/bugzilla>

Summary : The Linux kernel (the core of the Linux operating system)

Description :

The kernel package contains the Linux kernel (vmlinuz), the core of any

Linux operating system. The kernel handles the basic functions

of the operating system: memory allocation, process allocation, device

input and output, etc.

[root@RHEL01 ~

[root@RHEL01 ~]# uname -a

Linux RHEL01 2.6.18-8.el5 #1 SMP Fri Jan 26 14:15:14 EST 2007 x86_64 x86_64 x86_64 GNU/Linux

[root@RHEL01 ~]#

9. Install the new kernel using yum

Update the kernel package only:

[root@RHEL01 ~]# yum update kernel

Loading "rhnplugin" plugin

Setting up Update Process

Resolving Dependencies

--> Running transaction check

---> Package kernel.x86_64 0:2.6.18-128.1.14.el5 set to be installed

--> Finished Dependency Resolution

Dependencies Resolved

==========================================================

Package Arch Version Repository Size

==========================================================

Installing: kernel x86_64 2.6.18-128.1.14.el5 rhel-x86_64-server-5 17 M

Transaction Summary

==========================================================

Install 1 Package(s)

Update 0 Package(s)

Remove 0 Package(s)

Total download size: 17 M

Is this ok [y/N]: y

10.

Update all packages including the kernel:

[root@RHEL01 ~]# yum update

Loading "installonlyn" plugin

Loading "rhnplugin" plugin

Setting up Update Process

Setting up repositories

rhel-x86_64-server-5 100% |=========================| 1.3 kB 00:00

Reading repository metadata in from local files

primary.xml.gz 100% |=========================| 2.3 MB 00:01

################################################## 7019/7019

Resolving Dependencies

--> Populating transaction set with selected packages. Please wait.

---> Downloading header for redhat-lsb to pack into transaction set.

redhat-lsb-3.1-12.3.EL.x8 100% |=========================| 12 kB 00:00

---> Package redhat-lsb.x86_64 0:3.1-12.3.EL set to be updated

OUTPUT TRUNCATED

Transaction Summary

==========================================================

Install 33 Package(s)

Update 270 Package(s)

Remove 0 Package(s)

Total download size: 255 M

Is this ok [y/N]: y

11. Note: There may also be other available packages that need updating on the system (as seen above.) Yum should take care of the kernel as well as any other packages and associated dependencies on the system.

After answering yes to the continue question, yum will pull all the new packages and their dependencies from the RHN repository. It will then configure them, and update your boot loader to use the new kernel (requires a reboot).

12. The new kernel is now installed and ready for use. Reboot the system to boot into the new kernel image

[root@RHEL01 ~]# init 6

13.

14. Once the system has come back up, login and verify your running from the new kernel image

[root@RHEL01 ~]# uname -a

Linux REHL01 2.6.18-128.1.14.el5 #1 SMP Mon Jun 1 15:52:58 EDT 2009 x86_64 x86_64 x86_64 GNU/Linux

[root@RHEL01 ~]#

Linux Booting process explained

Knowing Linux booting process is an essential part of every Linux user/administration which will give you a clear picture of how Linux Operating system works. In this post we will see what happens when a Linux OS boots i.e. after powering on the machine to the user login prompt. Below image will give you clear idea what will happen in Linux booting process.

A quick view of booting sequence:

Power on

CPU jumps to BIOS

BIOS runs POST

Finds first bootable device

Load and execute MBR

Load OS

User prompt

This a rough idea what happens in Linux booting. Below are the detailed stages in Linux Booting process.

Stages of Booting:

1)System startup(Hardware )
2)Boot loader Stage 1 (MBR loading)
3)Boot loader Stage 2 (GRUB loader)
4)Kernel
5)INIT
6)User prompt

Stage 1: System startup

This is the first stage of booting process. When you power on/Restart your machine the power is supplied to SMPS (switched-mode power supply) which converts AC to DC. The DC power is supplied to all the devices connected to that machine such as Motherboard HDD's, CD/DVD-ROM, Mouse, keyboard etc. The most intelligent device in the computer is Processor(CPU), when supplied with power will start running its sequence operations stored in its memory. The first instruction it will run is to pass control to BIOS(Basic Input/Output System) to do POST(Power On Self Test). Once the control goes to BIOS it will take care of two things

· Run POST operation.

· Selecting first Boot device.

POST operation:

POST is a processes of checking hardware availability. BIOS will have a list of all devices which are present in previous system boot. In order to check if a hardware is available for the present booting or not it will send an electric plus to each and every device in the list that it already have. If an electrical pulse is returned from that device it will come to a conclusion the hardware is working fine and ready for use. If it does not receive a single from a particular device it will treat that device as faulty or it was removed from the system. If any new hardware is attached to the system it will do the same operation to find if its available or not. The new list will be stored in BIOS memory for next boot.

Selecting First Boot Device:

Once the POST is completed BIOS will have the list of devices available. BIOS memory will have the next steps details like what is the first boot device it has to select etc. It will select the first boot device and gives back the control to Processor(CPU). Suppose if it does not find first boot device, it will check for next boot device, if not third and so on. If BIOS do not find any boot device it will alert user stating "No boot device found".

Stage 2: MBR loading

Once the BIOS gives control back to CPU, it will try to load MBR of the first boot device(We will consider it as HDD). MBR is a small part of Hard Disk with just a size of 512 Bytes, I repeat its just 512 Bytes. This MBR resides at the starting of HDD or end of HDD depending on manufacturer.

What is MBR?

MBR(Master Boot recorder) is a location on disk which have details about

· Primary boot loader code(This is of 446 Bytes)

· Partition table information(64 Bytes)

· Magic number(2 Bytes)

Which will be equal to 512B (446+64+2)B.

Primary Boot loader code: This code provides boot loader information and location details of actual boot loader code on the hard disk. This is helpful for CPU to load second stage of Boot loader.

Partition table: MBR contains 64 bytes of data which stores Partition table information such as what is the start and end of each partition, size of partition, type of partition(Whether it's a primary or extended etc). As we all know HDD support only 4 partitions, this is because of the limitation of its information in MBR. For a partition to represent in MBR, it requires 16 Bytes of space in it so at most we will get 4 partitions.

Magic Number: The magic number service as validation check for MBR. If MBR gets corrupted this magic number is used to retrieve it. What to take backup of your MBR try this.

Once your CPU knows all these details, it will try to analyse them and read the first portion of MBR to load Second stage of Boot loader

Stage 3: Boot loader Stage 2 (GRUB loader)

Once the Bootloader stage 1 is completed and able to find the actual bootloader location, Stage 1 bootloader start second stage by loading Bootloader into memory. In this stage GRUB(Grand Unified Bootloader) which is located in the first 30 kilobytes of hard disk immediately following the MBR is loaded into RAM for reading its configuration and displays the GRUB boot menu (where the user can manually specify the boot parameters) to the user. GRUB loads the user-selected (or default) kernel into memory and passes control on to the kernel. If user do not select the OS, after a defined timeout GRUB will load the default kernel in the memory for starting it.

Stage 4: Kernel

Once the control is given to kernel which is the central part of all your OS and act as a mediator of hardware and software components. Kernel once loaded into to RAM it always resides on RAM until the machine is shutdown. Once the Kernel starts its operations the first thing it do is executing INIT process.

Stage 5: INIT

This is the main stage of Booting Process

init(initialization) process is the root/parent process of all the process which run under Linux/Unix. The first process it runs is a script at /etc/rc.d/rc.sysinit which check all the system properties, hardware, display, SElinux, load kernel modules, file system check, file system mounting etc. Based on the appropriate run-level, scripts are executed to start/stop various processes to run the system and make it functional. INIT process read /etc/inittab which is an initialization table which defines starting of system programs. INIT will start each run level one after the other and start executing scripts corresponds to that runlevel.

The script information is stored in different folders in /etc/ folder

/etc/rc0.d/ –Contain Start/Kill scripts which should be run in Runlevel 0
/etc/rc1.d/ –Contain Start/Kill scripts which should be run in Runlevel 1
/etc/rc2.d/ –Contain Start/Kill scripts which should be run in Runlevel 2
/etc/rc3.d/ –Contain Start/Kill scripts which should be run in Runlevel 3
/etc/rc4.d/ –Contain Start/Kill scripts which should be run in Runlevel 4
/etc/rc5.d/ –Contain Start/Kill scripts which should be run in Runlevel 5
/etc/rc6.d/ –Contain Start/Kill scripts which should be run in Runlevel 6

Once the initialization process completes mandatory run level and reach to default runlevel set in /etc/inittab, init process run one more file /etc/rc.local which are the last commands run in initialization process or even booting process. Once everything is completed the control is given back to the kernel

Stage 6: User prompt

This is actually not part of booting process but thought of including it here for better understating. Once the Kernel get the control it start multiple instances of "getty" which waits for console logins which spawn one's user shell process and gives you user prompt to login.

Runlevel Definition

A runlevel is a preset operating state on a Unix-like operating system.

A system can be booted into (i.e., started up into) any of several runlevels, each of which is represented by a single digit integer. Each runlevel designates a different system configuration and allows access to a different combination of processes (i.e.,instances of executing programs).

The are differences in the runlevels according to the operating system. Seven runlevels are supported in the standard Linux kernel (i.e., core of the operating system). They are:

0 - System halt; no activity, the system can be safely powered down.
1 - Single user; rarely used.
2 - Multiple users, no NFS (network filesystem); also used rarely.
3 - Multiple users, command line (i.e., all-text mode) interface; the standard runlevel for most Linux-based server hardware.
4 - User-definable
5 - Multiple users, GUI (graphical user interface); the standard runlevel for most Linux-based desktop systems.
6 - Reboot; used when restarting the system.

By default Linux boots either to runlevel 3 or to runlevel 5. The former permits the system to run all services except for a GUI. The latter allows all services including a GUI.

In addition to the standard runlevels, users can modify the preset runlevels or even create new ones if desired. Runlevels 2 and 4 are usually used for user defined runlevels.

The program responsible for altering the runlevel is init, and it can be called using thetelinit command. For example, changing from runlevel 3 to runlevel 5, which allows the GUI to be started, can be accomplished by the root (i.e., administrative) user by issuing the following command:

telinit 5

Booting into a different runlevel can help solve certain problems. For example, if a change made in the X Window System configuration on a machine that has been set up to boot into a GUI has rendered the system unusable, it is possible to temporarily boot into a console (i.e., all-text mode) runlevel (i.e., runlevels 3 or 1) in order to repair the error and then reboot into the GUI. The X Window System is a widely used system for managing GUIs on single computers and on networks of computers.

Likewise, if a machine will not boot due to a damaged configuration file or will not allow logging in because of a corrupted /etc/passwd file (which stores user names and other data about users) or because of a forgotten password, the problem can solved by first booting into single-user mode (i.e. runlevel 1).

The runlevel command can be used to find both the current runlevel and the previous runlevel by merely typing the following and pressing the Enter key:

/sbin/runlevel

The runlevel executable file (i.e., the ready-to-run form of the program) is typically located in the /sbin directory, which contains mostly administrative tools and which by default is not in the user's PATH (i.e., the list of directories in which the system searches for programs). Thus, it is usually necessary to type the full path of the command as shown above rather than just the name of the command itself.

The default runlevel for a system is specified in the /etc/inittab file, which will contain an entry such as id:3:initdefault: if the system starts in runlevel 3, orid:5:initdefault: if it starts in runlevel 5. This file can be easily (and safely) read with a command such as cat, i.e.,

cat /etc/inittab

As an alternative to telinit, the runlevel into which the system boots can be changed by modifying /etc/inittab manually with a text editor. However, it is generally easier and safer (i.e., less chance of accidental damage to the file) to use telinit. It is always wise to make a backup copy of /etc/inittab or any other configuration file before attempting to modify it manually.

The differences between Ext2 and Ext3 file systems are as follows.

Sl.No	EXT2	EXT3
Journaling	No journaling	Has journaling
Speed of file system(read-write)	Bit faster	Bit slower then ext2 file-system
Data corruption	File system may be corrupted due to unplanned reboots	Prevents file-system corruption
Way to recover data	Require fsck to recover data after unplanned reboot	Does not require(automatic file recovery is done at booting time)
Online file system growth.	By default there is no Online file system growth.	Online file system growth
Commands to format	mkfs.ext2 or mke2fs	mkfs.ext3 or mke2fs -j
Max file size(if block size is 1kB)	2GB	16GB
De-fragmentation	Easy and done with e2defrag	There is no data security when doing de-fragmentation.
Data accommodation	More data can be accommodated in a give space	Less date is written when compare to ext2 in a give space due to journaling.

Some FAQ’s:

1. What is Journaling?
Ans : In general, Journaling file systems avoid file system corruption by maintaining a journal. The journal is a special file that logs the changes destined for the file system in a circular buffer. At periodic intervals, the journal is committed to the file system. If a crash occurs, the journal can be used as acheckpoint to recover unsaved information and avoid corrupting of file system meta-data.
2.How many file systems supported by linux? and what are they?
Ans : Linux will supports : Btrfs, cifs, davfs, ext, ext2, ext3, ext4, exofs, hpfs, JFS minix, msdos, ncpfs, NiLFS(2), ntfs, nfs, proc, smbfs, iso9660, sysv, hpfs, affs, ufs, umsdos, vfat, xia, xfs, ZFS.

Note : Please add some more file systems which you people know in the comment section. So that i will update the post with new file systems.

3.What is the maximum size of a partition we can create using ext2 and ext3?
Ans : 4TB partition can be created in each ext2 and ext3 file system respectively.

4.What is the maximum size of a file we can create in ext2?
Ans : The maximum size what we can create is 2GB to 2TB. It depends on the block size we taken when we are formatting. If the block size is 1KB we can not create a file more than 2GB in ext2 file system.

5.What is “ext” in ext2 and ext3?
Ans : When linux first implemented the default file system is minixfs, in subsequent years it was replaced its successor file system called extended file-system. So the naming convention. So ext2 is second version of extended file system. Same explanation is given to ext3 and ext4.

6. Some points about ext4
a.It supports 64 bit storage limits, where as ext, ext2, ext3 are 32Bit storage limits and minix is 16Bit storage limit.
b.Have backward compatibility and performance is improved from lower version.
c.Extents are introduced (An extent is a range of contiguous physical blocks, improving large file performance and reducing fragmentation. A single extent inext4 can map up to 128MB of contiguous space with a 4KB block size).

What is disk quota?
Ans : Disk quota is nothing but restricting the disk-space usage to the users. We have to remember one thing when we are dealing with disk quota i.e Disk Quota can be applied only on disks/partitions not on files and folders.

So how we can implement disk quota?
Disk quota can be implemented in two ways

a. On INODE
b. On BLOCK

What is an INODE?
Ans : In Linux every object is consider as file, every file will be having an inode number associated and this is very much easy for computer to recognise where the file is located.

Inode stands for Index Node, and is the focus of all file activities in the UNIX file-system.
Each file has one inode that defines the file’s type (regular, directory, device etc),The location on disk, The size of the file, Access permissions, Access times.

Note that the file’s name is not stored in the inode.

So how to know what is your file Inode number?

Ans : Its just simple execute ls -i on your file.

ls -i xmls.txt

13662 xmls.txt

I think now you got what is INODE? Lets move on to BLOCK.

A block usually represents one least size on a disk, usually one block equal to 1kb. Some terms in Disk quota.

Soft limit : This is the disk limit where the user gets just a warning messagesaying that your disk quota is going to expire. This is just a warning, no restriction on data creation will occur at this point.

Hard limit : This is the disk limit where user gets error message, I repeat user gets error message stating that unable to create data.

Implementing QUOTA :
Step1 : Select/prepare the partition for quota, most of the time disk quota is implemented for restricting users not to create unwanted data on servers, so we will implement disk quota on /home mount point.

#vi /etc/fstab

Edit the /home mount point as follows
Before editing

/dev/hda2 /home ext3 defaults 0 0

after editing

/dev/hda2 /home ext3 defaults,usrquota 0 0

Step2 : Remounting the partition(this is done because the mount table should be updated to kernel). Other wise you can reboot the system too for updating of mount table, which is not preferred for live servers.

#mount -o remount,rw /home

Here -o specifies options, with remounting /home partition with read and write options.

Step3 : Creating quota database

#quotacheck -cu /home

The option -c for creating disk quota DB and u for user
Check for user database is created or not when you give ls /home you have to see auota.user file in /home directory,which contains user database.

Step4 : Switching on quota

#quotaon /home

Now get the report for default quota values for user surendra

#repquoata -a | grep surendra

surendra_anne --       4       0       0              1     0     0

surendra_a --             4       0       0              1     0     0

surendra_test --       16       0       0              4     0     0

Step5 : Now implementing disk quota for user phani on /home mount point(/dev/hda2)

#setquota -u surendra_anne 100 110 0 0 /dev/hda2

Step6 : Checking quota is implemented or not login to user surendra_anne and execute this command

#repquota -a

#quota

Step7 : Keep creating data, once 100MB is reached user will get an warning message saying, and when he reaches 110MB he can not create any more data.

Hint : To create a data file you can use seq command as below

#seq 1 10000 > test.txt

this command will create a file with 10000 lines with numbers in it.

Removing quota :
To do this one, all the users should log out from the system so better do it in runlevel one.

Step8 : Stop the disk quota

#quotaoff /home

Step9 : Removing quota database which is located /home

#rm /home/aquota.user

Step10 : Edit fstab file and remove usrdata from /home line

#vi /etc/fstab

Before editing

/dev/hda2 /home ext3 defaults,usrquota 0 0

After editing

/dev/hda2 /home ext3 defaults 0 0

Step11 : Remount the /home partition

#mount -o remount,rw /home

That’s it you are done with Disk Quota Implementation in Linux. Now test your self in creating Linux user disk quota on your own.

What are the file and file system size limitations for Red Hat Enterprise Linux?

Maximum (individual) file size
Filesystem	RHEL 3	RHEL 4	RHEL 5	RHEL 6	RHEL 7
EXT2/3	2TB	2TB	2TB	2TB	-
EXT4 [2]	n/a	n/a	16TB (5.4 or later) [2]	16TB	50TB
GFS1	2TB	16TB supported, 8EB limit	16TB supported, 8EB maximum	n/a	n/a
GFS2 [1]	n/a	n/a	25TB	100TB	100TB
XFS [3]	n/a	n/a	16TB	16TB	16TB
Maximum filesystem size
Filesystem	RHEL 3	RHEL 4	RHEL 5	RHEL 6	RHEL 7
EXT2/3	2TB	8TB	16TB (8TB in 5.0) [4]	16TB	-
EXT4 [2]	n/a	n/a	16TB (5.4 or later) [2]	16TB	50TB
GFS	2TB	16TB supported, 8EB limit	16TB supported, 8EB limit	n/a	n/a
GFS2 [1]	2TB	16TB	25TB [5]	100TB	100TB
XFS [3]	n/a	n/a	100TB	100TB	500TB
Units
Units are given in binary prefix.
TB = Terabyte = 2^40
EB = Exabyte = 2^60

Linux Commands to Monitor Memory Usage:

vmstat Monitor virtual memory
free Display amount of free and used memory in the system.
pmap Display/examine memory map and libraries (so). Usage: pmap pid
top Show top processes
sar -B Show statistics on page swapping.
time -v date Show system page size, page faults, etc of a process during execution. Note you must fully qualify the command as "/usr/bin/time" to avoid using the bash shell command "time".
cat /proc/sys/vm/freepages Display virtual memory "free pages".
One may increase/decrease this limit: echo 300 400 500 > /proc/sys/vm/freepages
cat /proc/meminfo Show memory size and usage.

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Thursday 30 January 2014

Go through Linux Interview Stuff

Stage 3: Boot loader Stage 2 (GRUB loader)

Stage 4: Kernel

Stage 5: INIT

Stage 6: User prompt

What are the file and file system size limitations for Red Hat Enterprise Linux?

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