System Run Levels

Main Run Level

• 0 » Shutdown (or halt) the system

• 1 » Single-user mode; usually aliased as s or S

• 6 » Reboot the system

Other Run Level

• 2 » Multiuser mode without networking

• 3 » Multiuser mode with networking

• 5 » Multiuser mode with networking and GUI

NOTE: Run-level 4 is undefined or not used/User-definable

• Command » init 0/1/6/2/3/5 & who -r » To check the run level details

Computer Boot Process

Linux Boot Process

Linux Boot Process (Newer Versions)

The systemd-analyze command

• Systemd is a system and service manager for Linux that manages services and system settings. It's the default initialization system for many Linux distributions.

• Boot by kernel, service, initrd

• It helps to identify slowest services

• Start time in blame list

• Optimizing boot performance

Some commands are-

1. systemd-analyze time

2. systemd-analyze blame

3. systemd-analyze blame --user

Message of the Day

• It will appear whenever you are connected.

• Message of the day file location » /etc/motd

  

  

Customize Message of the Day

• Message of the day is the first message users will see when they login to the Linux machine

• Steps:

  1. Create a new file in /etc/profile.d/motd.sh

  2. Add desired commands in motd.sh file

     

  3. Modify the /etc/ssh/sshd_config file to edit » #PrintMotd yes to no

     

     

  4. Restart sshd service

     

     

Storage

Local Storage

• RAM, HDD, SDD, etc.

DAS (Direct Attached Storage)

• CD/DVD, USB Flash Drive, external disk directly attached with USB or other cables

SAN (Storage Area Network)

• Storage attached through iSCSI or fiber cable

NAS (Network Attached Storage)

• Storage attached over network (TCP/IP)

• E.g., Samba, NFS etc.

Disk Partition

• Commands for disk partition

  1. df » df shows the available space on the file system which is set up on that device.

     

  2. fdisk -l » fdisk shows the total number of disk blocks

     

Add Disk and Creating Partition

• Purpose? » Out of space, Additional apps etc

• Commands » df and fdisk

Follow Below Steps-

# fdisk /dev/emcpowerp OR fdisk /dev/sdb m » n » p » 1 » enter » enter » w

Then format the new partition

# mkfs –t ext2 /dev/sdb1

OR

# mkfs.ext3 /dev/emcpowerL# OR mkfs.ext3 /dev/sdb1

Logical Volume Management (LVM)

• LVM allows disks to be combined together

Add Disk and Create LVM Partition

Steps-

fdisk /dev/sdc

n

p

Enter for first sector

Enter for last sector

p = print the partition table

t = change a partition's system id

L = type L to list all codes

8e = Partition type from Linux to Linux LVM

w

Create Physical Volume (PV) = pvcreate /dev/sdc1

Verify physical volume = pvdisplay

Create Volume Group (VG) = vgcreate oracle_vg /dev/sdc1

Verify Volume group = vgdisplay oracle_vg

Create Logical Volumes (LV) = lvcreate –n oracle_lv –size 2G oracle_vg

Verify logical volumes = lvdisplay

Format Logical Volumes = mkfs.xfs /dev/oracle_vg/oracle_lv

Create a new directory = mkdir /oracle

Mount the new file system = mount /dev/oracle_vg/oracle_lv /oracle

Verify = df –h

Extend Disk Using LVM

Go to your virtualization product (VMWare or Oracle Virtual Box)

Increase the disk space to desired number and then click ok

Now go to your Linux VM

Reboot the VM to have the system re-scan the newly added disk Or

cd /sys/class/scsi_disk/2:0:0:0

echo '1' > device/rescan

fdisk –l (To make sure the disk is increased)

Create a new partition

fdisk /dev/sdc

n (for new partition)

p (for primary partition)

2 (partition number, 2 or the new partition)

Enter

Enter

t (Label the new partition)

3 (Pick default value)

8e (This will make the filesystem as LVM)

w (Write)

# reboot or init 6

Note: The above procedure will create /dev/sdc2 partition

Extend the LVM group

pvdisplay (To see which group associated with which disk)

pvs (Info about physical volumes

vgdisplay oracle_vg (oracle_vg is the group name or you can simply run vgdisplay) On vgdisplay you will notice Free PE / Size at the bottom

pvcreate /dev/sdc2 (Initialize partition for use by LVM)

vgextend oracle_vg /dev/sdc2 (# = whichever partition was created above)

Run vgdisplay oracle_vg check (Free PE / Size). The second column is the right column as free. If it is in G convert that into M. e.g. 1G = 1024M

lvextend –L+1024M /dev/mapper/oracle_vg-oracle_lv

resize2fs /dev/mapper/oracle_vg-oracle_lv

OR

xfs_growfs /dev/mapper/oracle_vg-oracle_lv

Adding Swap Space

What is swap? – CentOS.org

Swap space in Linux is used when the amount of physical memory (RAM) is full. If the system needs more memory resources and the RAM is full, inactive pages in memory are moved to the swap space. While swap space can help machines with a small amount of RAM, it should not be considered a replacement for more RAM. Swap space is located on hard drives, which have a slower access time than physical memory

Recommended swap size = Twice the size of RAM

Lets say,

M = Amount of RAM in GB, and S = Amount of swap in GB, then

If M < 2

then S = M *2

Else S = M + 2

Commands

dd

mkswap

swapon or swapoff

Steps to Create Swap Space from Existing Disk

If you don’t have any additional disks, you can create a file somewhere on your filesystem, and use that file for swap space. The following dd command example creates a swap file with the name “newswap” under / directory with a size of 1024MB (1.0GB)

• # dd if=/dev/zero of=/newswap bs=1M count=1024

Where

if = read from FILE instead of stdin

of = write to FILE instead of stdout

bs = read and write BYTES at a

time count = total size of the file

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Change the permission of the swap file so that only root can access it.

# chmod go-r /newswap

OR

# chmod 0600 /newswap

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Make this file as a swap file using mkswap command.

# mkswap /newswap

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Enable the newly created newswap.

# swapon /newswap

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To make this swap file available as a swap area even after the reboot, add the following line to the /etc/fstab file.

# cat /etc/fstab

/newswap swap swap defaults 0 0

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Verify whether the newly created swap area is available for your use.

# swapon –s

# free –h

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If you don’t want to reboot to verify whether the system takes all the swap space mentioned in the /etc/fstab, you can do the following, which will disable and enable all the swap partition mentioned in the /etc/fstab

# swapoff -a

# swapon -a

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Thanks for going through this blog, Happy Learning !! 😁