Adding swap space to an attached volume via mkswap and swapon

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Attaching EBS volume to an AWS instance

To attach a volume to an existing AWS instance, please visit AWS : Attaching Amazon EBS volume to an instance.

We'll add a 2GB swap space to an instance.

Creating a swap space

Before we create a swap space, our system looks like this:

ubuntu@ip-172-31-25-188:~$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1      7.8G  6.9G  513M  94% /
none            4.0K     0  4.0K   0% /sys/fs/cgroup
udev            240M   12K  240M   1% /dev
tmpfs            49M  352K   49M   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            245M     0  245M   0% /run/shm
none            100M     0  100M   0% /run/user

Memory:

ubuntu@ip-172-31-25-188:~$ free
             total       used       free     shared    buffers     cached
Mem:        500212     485204      15008      12540      18276     151168
-/+ buffers/cache:     315760     184452
Swap:            0          0          0

After we attached an Amazon EBS volume (/dev/xvdf) to our instance, it is exposed as a block device:

ubuntu@ip-172-31-25-188:~$ lsblk
NAME    MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvda    202:0    0   8G  0 disk 
|-xvda1 202:1    0   8G  0 part /
xvdf    202:80   0   2G  0 disk

Let's make the volume as a swap space using mkswap command:

ubuntu@ip-172-31-25-188:~$ sudo mkswap -f /dev/xvdf
Setting up swapspace version 1, size = 2097148 KiB
no label, UUID=ecc29d61-f428-4f6c-9217-27d846a86b5e

Now, turn the swap on using swapon command:

ubuntu@ip-172-31-25-188:~$ sudo swapon /dev/xvdf

ubuntu@ip-172-31-25-188:~$ lsblk
NAME    MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvda    202:0    0   8G  0 disk 
|-xvda1 202:1    0   8G  0 part /
xvdf    202:80   0   2G  0 disk [SWAP]

We may get "busy device" error while we do swapon:

ubuntu@ip-172-31-25-188:~$ sudo swapon /dev/xvdf
swapon: /dev/xvdf: swapon failed: Device or resource busy

In that case, we can turn it off because somehow it's been on:

ubuntu@ip-172-31-25-188:~$ sudo swapoff /dev/xvdf
ubuntu@ip-172-31-25-188:~$ lsblk
NAME    MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvda    202:0    0   8G  0 disk 
-xvda1 202:1    0   8G  0 part /
xvdf    202:80   0   2G  0 disk

Then, turn the swap back on:

ubuntu@ip-172-31-25-188:~$ sudo swapon /dev/xvdf

ubuntu@ip-172-31-25-188:~$ lsblk
NAME    MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
xvda    202:0    0   8G  0 disk 
|-xvda1 202:1    0   8G  0 part /
xvdf    202:80   0   1G  0 disk [SWAP]

We can check to see if our swap file is being used with free. Here is the output from free on my AWS instance:

ubuntu@ip-172-31-25-188:~$ free
             total       used       free     shared    buffers     cached
Mem:        500212     485204      15008      12540      18276     151168
-/+ buffers/cache:     315760     184452
Swap:      2097148          0    2097148

To make the space available automatically when our system rebooted, we need to add the following line to /etc/fstab file:

/dev/xvdf   swap   swap   defaults  0 0

Creating a Swap File on CentOS 7

Unlike we've done in previous sections (swap on a attached volume), in this section, we'll create a swap file on an existing partition.

We can check if we already have some swap space available using swapon with the -s flag. It will display a summary of swap usage and availability on our storage device:

$ swapon -s
Filename	Type		Size	Used	Priority
/dev/xvdb       partition	262140	262140	-1

Another way of checking for swap space is with the free utility, which shows us the system's overall memory usage. We can see our current memory and swap usage (in megabytes):

$ free -m
              total        used        free      shared  buff/cache   available
Mem:            987         838          10          59         138          48
Swap:           255         255           0

Now we want to check our current drive usage:

$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/root        24G   14G  9.3G  60% /
devtmpfs        492M     0  492M   0% /dev
tmpfs           494M     0  494M   0% /dev/shm
tmpfs           494M   58M  437M  12% /run
tmpfs           494M     0  494M   0% /sys/fs/cgroup
tmpfs            99M     0   99M   0% /run/user/0
tmpfs            99M     0   99M   0% /run/user/1000

As we can see on the first line, our storage partition has 9.3G available, so we have a bit of space to work with.

Although there are many opinions about the appropriate size of a swap space, in general, an amount equal to or double the amount of memory on our system is a good starting point.

Let's a swap file within our filesystem. We will create a file called swapfile in our root (/) directory.

First, we must allocate the amount of space that we want for our swap file using fallocate command which creates a file of a preallocated size (2G):

$ sudo fallocate -l 2G /swapfile

After entering password to authorize sudo privileges, the swap file will be created. We can verify that the correct amount of space was reserved for swap by using ls:

$ ls -lh /swapfile
-rw-r--r-- 1 root root 2.0G Aug  1 18:37 /swapfile

Though our file is created, our system does not know that this is supposed to be used for swap. We need to tell our system to format this file as swap and then enable it.

We need to adjust the permissions on our swap file so that it isn't readable by anyone besides the root account. We can lock down the permissions with chmod:

$ sudo chmod 600 /swapfile
$ ls -lh /swapfile
-rw------- 1 root root 2.0G Aug  1 18:37 /swapfile

Let's tell our system to set up the swap space for use:

$ sudo mkswap /swapfile
Setting up swapspace version 1, size = 2097148 KiB
no label, UUID=0e257f17-8201-40fd-ae17-42245237810b

Now, our swap file is ready to be used as a swap space. We can start using it by typing the following command:

$ sudo swapon /swapfile

To verify that the procedure was successful, we can check whether our system reports swap space now:

$ swapon -s
Filename		Type		Size	Used	Priority
/dev/xvdb               partition	262140	262140	-1
/swapfile               file	       2097148	  9188	-2

The output assures us that we have a new swap file. We can use the free command again to make it sure:

$ free -m
              total        used        free      shared  buff/cache   available
Mem:            987         820           9          59         157          65
Swap:          2303         277        2026

Our swap has been set up successfully, and our operating system will begin to use it as needed.

Our swap file is enabled at the moment, but when we reboot, the server will not automatically enable the file for use. We can change that by modifying the fstab file, which is a table that manages filesystems and partitions.

Edit /etc/fstab file with sudo privileges, and add the following line to tell the operating system to automatically use the swap file that we just created:

# /etc/fstab
#
# Accessible filesystems, by reference, are maintained under '/dev/disk'
# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info
#
/dev/xvda       /               ext3    noatime,errors=remount-ro       0 1
/dev/xvdb       none            swap    sw                              0 0
/proc           /proc           proc    defaults                        0 0
tmpfs           /dev/shm        tmpfs   nodev,nosuid,noexec,mode=1777             0 0
devpts          /dev/pts        devpts  gid=5,mode=620                  0 0
sysfs           /sys            sysfs   defaults                        0 0
/swapfile       swap            swap    sw                              0 0

The server will check this file on each bootup, so the swap file will be ready for use from now on.

There are several command that checks swap space:

$ cat /proc/meminfo
...
SwapTotal:       2359288 kB
SwapFree:        2060708 kB
...

$ cat /proc/swaps
Filename		Type		Size	Used	Priority
/dev/xvdb               partition	262140	262140	-1
/swapfile               file		2097148	36440	-2


$ swapon -s
Filename		Type		Size	Used	Priority
/dev/xvdb               partition	262140	262140	-1
/swapfile               file	        2097148	36440	-2

Note that we can make another swapfile such as swapfile2:

$ sudo fallocate -l 2G /swapfile2
$ sudo chmod 600 /swapfile2
$ sudo mkswap /swapfile2
$ sudo swapon /swapfile2
$ echo "/swapfile2  swap  swap    sw  0 0" | sudo tee -a /etc/fstab

$ swapon -s
Filename	Type	   Size	        Used	Priority
/swapfile       file	   2097148	1951224	-2
/dev/xvdb       partition  262140	262140	-1
/swapfile2      file	   2097148	4	-3