Linux Swap Space Creation and Monitoring

Overview

This Post is intended to understand the swap creation, monitoring and extending in Redhat Linux.

Swap space is a restricted amount of physical memory that is allocated for use by the operating system 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 System Swap Space
In years past, the recommended amount of swap space increased linearly with the amount of RAM in the system. But because the amount of memory in modern systems has increased into the hundreds of gigabytes, it is now recognized that the amount of swap space that a system needs is a function of the memory workload running on that system. However, given that swap space is usually designated at install time, and that it can be difficult to determine beforehand the memory workload of a system, Redhat recommend determining system swap using the following table.

Amount of RAM in the System Recommended Amount of Swap Space
4GB of RAM or less a minimum of 2GB of swap space
4GB to 16GB of RAM a minimum of 4GB of swap space
16GB to 64GB of RAM a minimum of 8GB of swap space
64GB to 256GB of RAM a minimum of 16GB of swap space
256GB to 512GB of RAM a minimum of 32GB of swap space

Note : On most distributions of Linux, it is recommended that you set swap space while installing the operating system

 

How to Monitor Swap Space

We shall look at different commands and tools that can help you to monitor your swap space usage in your Linux systems as follows

Using the swapon Command

To view all devices marked as swap in the /etc/fstab file you can use the –all option. Though devices that are already working as swap space are skipped

If you want to view a summary of swap space usage by device, use the – summary (swapon –s) option.

[[email protected] ~]# swapon –summary
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       2097148 0       -1
[[email protected] ~]# swapon -s
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       2097148 0       -1
Note :- Use –help option to view more options and information.
Using /proc/swaps

The /proc filesystem is a process information pseudo-file system. It actually does not contain ‘real’ files but runtime system information, for example system memory, devices mounted, hardware configuration and many more.

[[email protected] ~]# cat /proc/swaps

Filename                                Type            Size    Used    Priority

/dev/dm-1                               partition       2097148 0       -1

[[email protected] ~]#

Using ‘free’ Command
The free command is used to display the amount of free and used system memory. Using the free command with -h option, which displays output in a human readable format.
[[email protected] ~]# free -h
              total        used        free      shared  buff/cache   available
Mem:           7.6G        674M        6.5G        9.8M        507M        6.7G
Swap:          2.0G          0B        2.0G
 Using top Command
To check swap space usage with the help of ‘top’ command
Using the vmstat Command
This command is used to display information about virtual memory statistics
[[email protected] ~]# vmstat
procs ———–memory———- —swap– —–io—- -system– ——cpu—–
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0      0 6791708   2784 516484    0    0     7     0   24   23  0  0 100  0  0
ADDING SWAP SPACE
Sometimes it is necessary to add more swap space after installation
You have three options: create a new swap partition, create a new swap file, or extend swap on an existing LVM2 logical volume. It is recommended that you extend an existing logical volume
Extending Swap on an LVM2 Logical Volume
To extend an LVM2 swap logical volume(suppose /dev/mapper/centos-swap is our swap volume)
1. Disable swapping for the associated logical volume:
[[email protected] ~]# swapoff -v /dev/mapper/centos-swap
swapoff /dev/mapper/centos-swap
[[email protected] ~]# swapon -s
2. Resize the LVM2 logical volume by 256 MB
 [[email protected] ~]# lvresize /dev/mapper/centos-swap -L +256M
  Size of logical volume centos/swap changed from 2.00 GiB (512 extents) to 2.25 GiB (576 extents).
  Logical volume centos/swap successfully resized.
3. Format the new swap space
[[email protected] ~]# mkswap /dev/centos/swap
mkswap: /dev/centos/swap: warning: wiping old swap signature.
Setting up swapspace version 1, size = 2359292 KiB
no label, UUID=5e487401-9ae0-4e1d-adff-2346edfc6244
4. Enable the extended logical volume
[[email protected] ~]# swapon -va
swapon /dev/mapper/centos-swap
swapon: /dev/mapper/centos-swap: found swap signature: version 1, page-size 4, same byte order
swapon: /dev/mapper/centos-swap: pagesize=4096, swapsize=2415919104, devsize=2415919104
5. Test that the logical volume has been extended properly
[[email protected] ~]# free -h
              total        used        free      shared  buff/cache   available
Mem:           7.6G        677M        6.5G        9.8M        507M        6.7G
Swap:          2.2G          0B        2.2G
[[email protected] ~]# swapon -s
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       2359292 0       -1
Creating an LVM2 Logical Volume for Swap
To add a swap volume group (suppose /dev/centos/swap2 is the new volume)
1. Create the LVM2 logical volume of size 256 MB
[[email protected] ~]# lvcreate centos -n swap2 -L 256M
  Logical volume “swap2” created.
2. Format the new swap space
[[email protected] ~]# mkswap /dev/centos/swap2
Setting up swapspace version 1, size = 262140 KiB
no label, UUID=6ea40455-47a0-46bf-844e-ec0ebd4a4e6a
3. Add the following entry to the /etc/fstab file
/dev/mapper/centos-swap2 swap                    swap    defaults        0 0
4. Enable the extended logical volume
[[email protected] ~]# swapon –va
swapon /dev/mapper/centos-swap2
swapon: /dev/mapper/centos-swap2: found swap signature: version 1, page-size 4, same byte order
swapon: /dev/mapper/centos-swap2: pagesize=4096, swapsize=268435456, devsize=268435456
5. Verify the swap space
[[email protected] ~]# swapon -s
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       2097148 0       -1
/dev/dm-3                               partition       262140  0       -2
Creating a Swap File
To Add a swap file
1. Determine the size of the new swap file in megabytes and multiply by 1024 to determine the number of blocks. For example, the block size of a 64 MB swap file is 65536.
2. At a shell prompt as root, type the following command with count being equal to the desired block size:
[[email protected] ~]# dd if=/dev/zero of=/swapfile bs=1024 count=65536
65536+0 records in
65536+0 records out
67108864 bytes (67 MB) copied, 0.0893063 s, 751 MB/s
[[email protected] ~]# ls -ld /swapfile
-rw-r–r–. 1 root root 67108864 May 17 16:38 /swapfile
[[email protected] ~]# du -sh /swapfile
64M     /swapfile
3. Change the permissions of the newly created file
[[email protected] ~]# chmod 0600 /swapfile
4. Setup the swap file with the command
[[email protected] ~]# mkswap /swapfile
Setting up swapspace version 1, size = 65532 KiB
no label, UUID=8a404550-e8a3-4f2b-9daf-137fc34f7b6d
5. Edit /etc/fstab and enable the newly added swap space
/swapfile          swap            swap    defaults        0 0
[[email protected] ~]# swapon -va
swapon /swapfile
swapon: /swapfile: found swap signature: version 1, page-size 4, same byte order
swapon: /swapfile: pagesize=4096, swapsize=67108864, devsize=67108864
6. Verify the swap space created.
[[email protected] ~]# swapon -s
Filename                                Type            Size    Used    Priority
/dev/dm-1                               partition       2097148 0       -1
/dev/dm-3                               partition       262140  0       -2
/swapfile                               file    65532   0       -3
Hope this has helped you ..
Thanks!!!!

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