Linux Disk Management: Day 04

Topics

Recap

  • Umask usage( to control the default permission)

  • Log into the Vagrant virtual machine using the default SSH private key.

    .vagrant/machines/<machine-name>/<provider>/private_key
ssh -i <privatekey> <user-name>/ip

  • Convert the private key .pem file into ppk to login with putty.

  • Login to 3rd server but private key is on 1st server

    • 🔐 SSH into a Third Server When Private Key Is on First Server (Using Agent Forwarding)

    🧭 Scenario

    You want to SSH into Server C from Server A, but the SSH private key is only on your local machine (Laptop). You don’t want to copy the key to Server A for security reasons.

    ✅ Solution: SSH Agent Forwarding

    This allows you to forward your SSH key securely from your Laptop → Server A → Server C without ever copying the private key.

    📝 Steps

    • Start SSH Agent on Your Local Machine
    eval "$(ssh-agent -s)"
ssh-add /path/to/your/private_key
    • Check loaded keys:
     ssh-add -l
    • SSH into Server A with Agent Forwarding Enabled
     ssh -A user@serverA
    • From Server A, SSH into Server C
    ssh user@serverC

  • Test the port forward on Virtualbox

  • Test the port forward on Putty

📂 1. Listing Disks and Partitions

View all block devices:

lsblk

Show detailed partition table:

fdisk -l

View disk usage:

df -h

View inode usage:

df -i

🧱 2. Partitioning Disks

Using fdisk (for MBR-style partitions):

sudo fdisk /dev/sdX
# n -> new partition
# p -> primary
# w -> write changes

🛠️ 3. Creating Filesystems

Create ext4 filesystem:

sudo mkfs.ext4 /dev/sdX1

Create xfs filesystem:

sudo mkfs.xfs /dev/sdX1

📦 4. Mounting and Unmounting Disks

Create a mount point and mount:

sudo mkdir /mnt/data
sudo mount /dev/sdX1 /mnt/data

Unmount:

sudo umount /mnt/data

View mounted filesystems:

mount | grep sd

🔁 5. Persistent Mounting with /etc/fstab

Get UUID:

blkid /dev/sdX1

Edit /etc/fstab:

UUID=xxxx-xxxx  /mnt/data  ext4  defaults  0  2

Then run:

sudo mount -a   # To verify fstab is valid

🔍 6. Disk Usage Analysis

Using du:

du -sh /home/user

Using ncdu (interactive tool):

sudo apt install ncdu   # or yum install ncdu
ncdu /

🔒 7. Checking & Repairing Filesystems

ext4 check:

sudo fsck /dev/sdX1

Must unmount first or run in recovery mode.

🔒 Introduction to LVM (Logical Volume Management)

LVM (Logical Volume Management) is a flexible and advanced way to manage disk storage in Linux. It allows you to create, resize, and manage storage volumes dynamically, as opposed to traditional partitioning. LVM abstracts the storage devices, giving you more control and flexibility over disk management.

Key Concepts:

  1. Physical Volumes (PV): These are the actual hard drives or partitions that are initialized for use by LVM. A physical volume can be any storage device, such as a disk or a partition.
  2. Volume Groups (VG): A volume group is a collection of physical volumes. It pools storage from the physical volumes, making it easier to allocate storage dynamically.
  3. Logical Volumes (LV): Logical volumes are created within volume groups. These are the virtual storage units that users can use like a regular partition or disk, where file systems are created and data is stored.

Advantages of LVM:

  • Dynamic Volume Resizing: Easily resize logical volumes and file systems without data loss.
  • Snapshots: Create backups of volumes at any point in time.
  • Better Disk Utilization: You can combine multiple physical disks into a single volume group for better utilization and flexibility.
  • Striping and Mirroring: LVM supports RAID-like features such as striping (for performance) and mirroring (for redundancy).

Now that we have a basic understanding of LVM, let’s look at some common LVM commands used to manage storage volumes.

Install LVM if commands are not available

yum install lvm2

🔒 LVM Commands

1️⃣ List Physical Volumes (PV)

pvs

This command lists all physical volumes in the system, showing basic information about each PV.

2️⃣ Display Detailed Information about a Physical Volume

pvdisplay /dev/sda

Displays detailed information about a specific physical volume.

3️⃣ Create a Physical Volume

pvcreate /dev/sda

Initialize a physical volume on a device (e.g., /dev/sda), so it can be added to a volume group.

4️⃣ Extend a Physical Volume

pvresize /dev/sda

Resize a physical volume, which is useful if you’ve expanded the underlying disk.

5️⃣ List Volume Groups (VG)

vgs

Shows a list of all volume groups in the system.

6️⃣ Display Detailed Information about a Volume Group

vgdisplay vg_name

Displays detailed information about a specific volume group, including the total size, free space, and more.

7️⃣ Create a Volume Group

vgcreate vg_name /dev/sda /dev/sdb

Creates a new volume group by adding physical volumes (e.g., /dev/sda and /dev/sdb) to the group.

8️⃣ Extend a Volume Group by Adding More Physical Volumes

vgextend vg_name /dev/sdc

Adds a new physical volume to an existing volume group to increase its available storage.

9️⃣ Reduce the Size of a Volume Group

vgreduce vg_name /dev/sdb

Removes a physical volume from a volume group.

🔟 Create a Logical Volume (LV)

lvcreate -n lv_name -L 10G vg_name

Create a new logical volume named lv_name with a size of 10GB in the specified volume group vg_name.

1️⃣1️⃣ Display Information about Logical Volumes

lvdisplay /dev/vg_name/lv_name

Shows detailed information about a specific logical volume.

1️⃣2️⃣ Extend a Logical Volume

lvextend -L +10G /dev/vg_name/lv_name

Extend a logical volume by 10GB.

1️⃣3️⃣ Resize a Logical Volume (after extending)

resize2fs /dev/vg_name/lv_name

Resize the file system on a logical volume after it has been extended.

1️⃣4️⃣ Remove a Logical Volume

lvremove /dev/vg_name/lv_name

Removes a specified logical volume.

1️⃣5️⃣ Remove a Volume Group

vgremove vg_name

Removes a specified volume group.

1️⃣6️⃣ Remove a Physical Volume

pvremove /dev/sda

Removes a physical volume, typically after removing it from the volume group.

1️⃣7️⃣ List All Available LVM Commands

lvm

Launches the interactive LVM command-line interface.

1️⃣8️⃣ Display LVM System Information

lvm pvscan

Scans for physical volumes and shows details about their status.

2️⃣0️⃣ Activate a Volume Group

vgchange -ay vg_name

Activates the specified volume group, making it available for use.

2️⃣1️⃣ Deactivate a Volume Group

vgchange -an vg_name

Deactivates a volume group, making it unavailable for use.

🔒 Swap Partitions in Linux

A swap partition is used by Linux to extend the available memory by using disk space. When the physical RAM is full, the system moves some of the inactive pages in RAM to swap space. It helps prevent the system from running out of memory and crashing. Swap partitions are particularly useful on systems with limited physical memory (RAM) or for systems running memory-intensive applications.

Key Points About Swap:

  1. Swap Partition: A dedicated disk partition used specifically for swap space.
  2. Swap File: A regular file on an existing file system used to create swap space.
  3. Swapiness: A kernel parameter that determines how aggressively the kernel will use swap space. A value of 0 means the system will avoid swapping, and a value of 100 means the system will swap aggressively.
  4. Performance Impact: Swap space is much slower than physical RAM, so it’s crucial not to rely too much on it. Excessive swapping (swapping too often) can slow down system performance.

Swap Space Advantages:

  • Prevents Out of Memory Errors: Swap provides a backup when the system runs out of RAM.
  • Improved System Stability: Helps prevent crashes due to memory exhaustion.
  • Memory Management: Allows Linux to keep rarely used parts of memory in swap and free up physical RAM for active processes.

🔒 Commands Related to Swap Partitions

1️⃣ Check Current Swap Usage

swapon -s

This command displays a summary of the current swap space in use on the system.

2️⃣ Show Swap Information

free -h

Displays the memory and swap usage in a human-readable format, including total, used, and available swap space.

3️⃣ Create a Swap Partition

fdisk /dev/sda

You can use fdisk to create a swap partition on a disk. This will involve creating a new partition and changing its type to swap.

  • In fdisk, select the disk and choose the n option to create a new partition.
  • Then, use the t command to change the partition type to 82 (Linux swap).

4️⃣ Format a Partition as Swap

mkswap /dev/sda3

Formats a partition (/dev/sda3 in this case) as swap space.

5️⃣ Enable the Swap Partition

swapon /dev/sda3

Activates the swap partition (/dev/sda3) for use by the system.

6️⃣ Disable a Swap Partition

swapoff /dev/sda3

Disables a swap partition, freeing up the space for other uses.

7️⃣ Check Swap Usage (Detailed)

cat /proc/meminfo | grep Swap

Shows detailed information about swap space usage from the system’s memory information.

8️⃣ Add Swap Space Permanently to /etc/fstab

echo '/dev/sda3 none swap sw 0 0' | sudo tee -a /etc/fstab

This command adds the swap partition (/dev/sda3) to /etc/fstab so that it is automatically enabled at boot time.

9️⃣ Create a Swap File

dd if=/dev/zero of=/swapfile bs=1M count=4096

This command creates a swap file (/swapfile) of 4GB (4096 MB).

🔟 Change Permissions on the Swap File

chmod 600 /swapfile

Sets the appropriate permissions for the swap file to ensure that only root can access it.

1️⃣1️⃣ Make the Swap File

mkswap /swapfile

Formats the swap file (/swapfile) to make it usable as swap space.

1️⃣2️⃣ Activate the Swap File

swapon /swapfile

Enables the swap file (/swapfile) for use.

1️⃣3️⃣ Remove the Swap File

swapoff /swapfile
rm /swapfile

Disables and removes the swap file from the system.

1️⃣4️⃣ Show the Current Swappiness Setting

cat /proc/sys/vm/swappiness

Displays the current swappiness value, which controls how often swap space is used.

1️⃣5️⃣ Set the Swappiness Value

sysctl vm.swappiness=10

Sets the swappiness value to 10 (lower means less aggressive swapping).

1️⃣6️⃣ Make Swappiness Value Persistent

echo "vm.swappiness = 10" | sudo tee -a /etc/sysctl.conf

Adds the swappiness setting to /etc/sysctl.conf for persistence across reboots.

🔒 Conclusion on Swap

Swap partitions and swap files are essential for managing memory on Linux systems, especially when running applications that consume a lot of memory. However, it’s important to monitor swap usage to avoid performance degradation due to excessive swapping. It’s best to have enough physical RAM to reduce reliance on swap space.

📘 Summary

  • lsblk, fdisk -l, df -h – View disk info
  • fdisk, parted – Create partitions
  • mkfs.ext4, mkfs.xfs – Format partitions
  • mount, umount, fstab – Manage mounts
  • fsck – Repair filesystems

Linux disk management is critical for system setup, storage provisioning, and data integrity.

🧾 RHEL OS Logging Overview

RHEL 8 uses systemd-journald for system logs and rsyslog for traditional log file storage. Understanding both is essential for troubleshooting and auditing.

📒 Systemd Journal Logs (journald)

🔍 View Logs

  • Show all logs
journalctl
  • Show logs for a specific service
journalctl -u sshd
  • Follow logs in real time
journalctl -f
  • Show logs since boot
journalctl -b
  • Show logs from today
journalctl --since today
  • Show logs between time ranges
journalctl --since "2024-07-01" --until "2024-07-10"