Disk and space management is an essential knowledge of a sysadmin. It is his or her everyday job to handle disk issues. As part of the RHCSA exam preparation, we will learn how to add new space of various types to the system, using the tools provided by RHEL8. We already covered many of these tasks, and in this tutorial we will focus on adding new space without harming the data contained in the system.

In this tutorial you will learn:
  • How to add new partitions to RHEL8
  • How to add new logical volumes to RHEL8
  • How to add swap to RHEL8
Adding swap space to the system
Adding swap space to the system.

Software Requirements and Conventions Used

Software Requirements and Linux Command Line Conventions
Category Requirements, Conventions or Software Version Used
System Red Hat Enterprise Linux 8.1
Software util-linux 2.32.1
Other Privileged access to your Linux system as root or via the sudo command.
Conventions # - requires given linux commands to be executed with root privileges either directly as a root user or by use of sudo command
$ - requires given linux commands to be executed as a regular non-privileged user

How to add new partitions to RHEL8


SUBSCRIBE TO NEWSLETTER
Subscribe to Linux Career NEWSLETTER and receive latest Linux news, jobs, career advice and tutorials.


We already covered the technical steps to create a partition in the partitioning tutorial for RHCSA, so here all that needs to be made clear is to think twice before partitioning. fdisk may ask if you are sure about a modification, but after that it will rewrite the partition table, which may result the data already on disk becomes inaccessible. Always create a backup if the data is needed.

How to add new logical volumes to RHEL8

Creating a new logical volume is covered in the logical volume tutorial for RHCSA. Logical volumes give system storage the flexibility that single disks alone can't provide. What need to be outlined here is that we need to build our LVM setup from the bottom, and if we need to extend it, we always need to check if we have enough space in the underlying physical volumes.

How to add swap to RHEL8

Adding swap space to the system online

Swap is a special disk space which is used by the operating system to write memory pages to. Memory contents that are not used for some time can be written to this swap partition, and thus the memory can be used by other programs. If the system is running low on free memory, swap will help solve the problem. But if the system need to use more memory it actually has, intense read/write occurs on the swap partition (which is called "swapping"), which will slow down the overall system, to the point where it becomes less and less responsive.



This is something that should be avoided, even a desktop is hard to use while swapping, and imagine a production server that runs an OLTP database, which suddenly starts swapping. While this intense swapping is a dreaded situation, using the swap space is a good thing in general - it helps run operations faster. If we are about to add more swap space to the system, we have two options: we can create a new swap partition, or we can create a file in the filesystem that will serve as swap space. We will cover both cases.

Creating a new swap partition

To create a swap partition, we need an empty partition on one of our disks, without a filesystem. In our test setup we have a 2 GB empty disk seen by the system as /dev/sdb:

# fdisk -l /dev/sdb 
Disk /dev/sdb: 2 GiB, 2147483648 bytes, 4194304 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes

Creating a swap partition is only one command away, called mkswap.

# mkswap /dev/sdb 
Setting up swapspace version 1, size = 2 GiB (2147479552 bytes)
no label, UUID=34aa2332-0514-42ab-9635-1fd6b922d213

As with all disk operations, check twice before proceeding. Reformatting a partition to swap destroys any content it held before. The mkswap utility is providing us the new partition's UUID, which will be needed for mounting.



Next we'll create a swapfile in the filesystem, which we'll also use as swap. Because our test environtment uses xfs filesystem, we'll need to prepare the file using dd.

# dd if=/dev/zero of=swapfile count=2048000
2048000+0 records in
2048000+0 records out
1048576000 bytes (1.0 GB, 1000 MiB) copied, 7.91227 s, 133 MB/s

We copied the default 512 bytes from /dev/zero two million times to the file called "swapfile", effectively filling the file with zeros to the size of around 1 GB. This file in turn can be used to create swap:

# mkswap swapfile 
mkswap: swapfile: insecure permissions 0644, 0600 suggested.
Setting up swapspace version 1, size = 1000 MiB (1048571904 bytes)
no label, UUID=8dc7aa71-524c-4d2b-bbb3-5b9fbbfb3327

Our initial swap space on the test environment is 1.5 GB:

# free -m | grep Swap
Swap:          1535         482        1053

Which is provided by the device /dev/dm-1:

# cat /proc/swaps 
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       1572860 492984  -2

To make our new swap spaces usable to the system with the swapon command. Running it without arguments provide data of the swap already available:


SUBSCRIBE TO NEWSLETTER
Subscribe to Linux Career NEWSLETTER and receive latest Linux news, jobs, career advice and tutorials.


# swapon 
NAME      TYPE      SIZE USED PRIO
/dev/dm-1 partition 1.5G 478M   -2

Adding our swapfile as argument enables the file as swap. In this example the file is in the /root directory, where the command is issued from, so no path need to be specified.

# swapon swapfile

The free command shows the increased swap space:

# free -m
              total        used        free      shared  buff/cache   available
Mem:            981         615         121           7         243         216
Swap:          2535         477        2058

We can turn on the /dev/sdb swap partition by UUID:

# swapon UUID=34aa2332-0514-42ab-9635-1fd6b922d213

And again our swap space is increased:

# free -m
              total        used        free      shared  buff/cache   available
Mem:            981         617         119           7         243         215
Swap:          4583         477        4106

And the swapon command also shows our new devices:

# swapon
NAME           TYPE       SIZE USED PRIO
/dev/dm-1      partition  1.5G 474M   -2
/root/swapfile file      1000M   0B   -3
/dev/sdb       partition    2G   0B   -4


And with this we have successfully added new swap space to our system. These changes aren't permanent at this stage, after reboot they will not be recognized as swap. To make use of these swap spaces after reboot, we'll need to add two entries into the /etc/fstab file that points to these devices, so the system can recognize and mount them at startup.

# tail -n 2 /etc/fstab
UUID=34aa2332-0514-42ab-9635-1fd6b922d213       swap                    swap    defaults        0 0
/root/swapfile  swap                    swap    defaults        0 0

Exercises

  1. After creating a swapfile and adding it to the /etc/fstab, delete it, and reboot your system. Will there be errors in the logfiles and the swapon output?
  2. After adding swap with either a file or partition, try turning the old partition off with swapoff. Don't do this on production systems!
  3. Try mkswap on a partition with a filesystem.
FIND LATEST LINUX JOBS on LinuxCareers.com
Submit your RESUME, create a JOB ALERT or subscribe to RSS feed.
LINUX CAREER NEWSLETTER
Subscribe to NEWSLETTER and receive latest news, jobs, career advice and tutorials.
DO YOU NEED ADDITIONAL HELP?
Get extra help by visiting our LINUX FORUM or simply use comments below.