Adding and Managing Ubuntu 20.04 Swap Space

An important part of maintaining the performance of an Ubuntu system involves ensuring that adequate swap space is available comparable to the memory demands placed on the system. The goal of this chapter, therefore, is to provide an overview of swap management on Ubuntu.

1.1  What is Swap Space?

Computer systems have a finite amount of physical memory that is made available to the operating system. When the operating system begins to approach the limit of the available memory it frees up space by writing memory pages to disk. When any of those pages are required by the operating system they are subsequently read back into memory. The area of the disk allocated for this task is referred to as swap space.

1.2  Recommended Swap Space for Ubuntu

The amount of swap recommended for Ubuntu depends on a number of factors including the amount of memory in the system, the workload imposed on that memory and whether the system is required to support hibernation. The current guidelines for Ubuntu swap space are as follows:

Amount of installed RAMRecommended swap spaceRecommended swap space if hibernation enabled

Table 36-1

For systems with memory configurations exceeding 24GB refer to the following web page for swap space guidelines:

When a system enters hibernation, the current system state is written to the hard disk and the host machine is powered off. When the machine is subsequently powered on, the state of the system is restored from the hard disk drive. This differs from suspension where the system state is stored in RAM. The machine then enters a sleep state whereby power is maintained to the system RAM while other devices are shut down.

1.3  Identifying Current Swap Space Usage

The current amount of swap used by an Ubuntu system may be identified in a number of ways. One option is to output the /proc/swaps file:

# cat /proc/swaps
Filename               Type       Size        Used     Priority
/dev/dm-1              partition  4169724     41484    -2

Alternatively, the swapon command may be used:

# swapon
/dev/dm-1 partition   4G 40.5M   -2

To view the amount of swap space relative to the overall available RAM, the free command may be used:

# free
        total        used        free      shared  buff/cache   available
Mem:  4035436     1428276     2224596       21968      382564     2360172
Swap: 4169724       41484     4128240

1.4  Adding a Swap File to an Ubuntu System

Additional swap may be added to the system by creating a file and assigning it as swap. Begin by creating the swap file using the dd command. The size of the file can be changed by adjusting the count= variable. The following command-line, for example, creates a 2.0 GB file:

# dd if=/dev/zero of=/newswap bs=1024 count=2000000
2000000+0 records in
2000000+0 records out
2048000000 bytes (2.0 GB, 1.9 GiB) copied, 3.62697 s, 565 MB/s

Before converting the file to a swap file, it is important to make sure the file has secure permissions set:

# chmod 0600 /newswap

Once a suitable file has been created, it needs to be converted into a swap file using the mkswap command:

# mkswap /newswap
Setting up swapspace version 1, size = 1.9 GiB (2047995904 bytes)
no label, UUID=4ffc238d-7fde-4367-bd98-c5c46407e535

With the swap file created and configured it can be added to the system in real-time using the swapon utility:

# swapon /newswap

Re-running swapon should now report that the new file is now being used as swap:

# swapon
/dev/dm-1 partition   4G   0B   -2
/newswap  file       1.9G   0B   -3

The swap space may be removed dynamically by using the swapoff utility as follows:

# swapoff /newswap

Finally, modify the /etc/fstab file to automatically add the new swap at system boot time by adding the following line:

/newswap    swap    swap   defaults 0 0

1.5  Adding Swap as a Partition

As an alternative to designating a file as swap space, entire disk partitions may also be designated as swap. The steps to achieve this are largely the same as those for adding a swap file. Before allocating a partition to swap, however, make sure that any existing data on the corresponding filesystem is either backed up or no longer needed and that the filesystem has been unmounted.

Assuming that a partition exists on a disk drive represented by /dev/sdb1, for example, the first step would be to convert this into a swap partition, once again using the mkswap utility:

# mkswap /dev/sdb1
mkswap: /dev/sdb1: warning: wiping old xfs signature.
Setting up swapspace version 1, size = 8 GiB (8587833344 bytes)
no label, UUID=a899c8ec-c410-4569-ba18-ddea03370c7f 

Next, add the new partition to the system swap and verify that it has indeed been added:

# swapon /dev/sdb1
# swapon
/dev/dm-1 partition   4G   0B   -2
/dev/sdb1 partition   8G   0B   -3

Once again, the /etc/fstab file may be modified to automatically add the swap partition at boot time as follows:

/dev/sdb1 defaults 0 0

1.6  Adding Space to an Ubuntu LVM Swap Volume

On systems using Logical Volume Management, an alternative to adding swap via file or disk partition is to extend the logical volume used for the swap space.

The first step is to identify the current amount of swap available and the volume group and logical volume used for the swap space using the lvdisplay utility (for more information on LVM, refer to the chapter entitled “Adding a New Disk to an Ubuntu Volume Group and Logical Volume”):

# lvdisplay
  --- Logical volume ---
  LV Path                /dev/vgubuntu/swap_1
  LV Name                swap_1
  VG Name                vgubuntu
  LV UUID                nJPip0-Q6dx-Mfe3-4Aao-gWAa-swDk-7ZiPdP
  LV Write Access        read/write
  LV Creation host, time ubuntu, 2020-01-13 13:16:18 -0500
  LV Status              available
  # open                 2
  LV Size                5.00 GiB
  Current LE             1280
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:1

Clearly the swap resides on a logical volume named swap_1 which is part of the volume group named vgubuntu. The next step is to verify if there is any space available on the volume group that can be allocated to the swap volume:

# vgs
  VG        #PV #LV #SN Attr   VSize   VFree 
  vgubuntu   2   3   0 wz--n- 197.66g <22.00g

If the amount of space available is sufficient to meet additional swap requirements, turn off the swap and extend the swap logical volume to use as much of the available space as needed to meet the system’s swap requirements:

# lvextend -L+8GB /dev/vgubuntu/swap_1
    Logical volume ubuntu_vg/swap_1 successfully resized.

Next, reformat the swap volume and turn the swap back on:

# mkswap /dev/vgubuntu/swap_1
mkswap: /dev/vgubuntu/swap_1: warning: wiping old swap signature.
Setting up swapspace version 1, size = 12 GiB (12754874368 bytes)
no label, UUID=241a4818-e51c-4b8c-9bc9-1697fc2ce26e
# swapon /dev/vgubuntu/swap_1

Having made the changes, check that the swap space has increased:

# swapon
/dev/dm-1 partition  12G   0B   -2

1.7  Adding Swap Space to the Volume Group

In the above section we extended the swap logical volume to use space that was already available in the volume group. If no space is available in the volume group then it will need to be added before the swap can be extended.

Begin by checking the status of the volume group:

# vgs
  VG        #PV #LV #SN Attr   VSize   VFree
  vgubuntu   1   2   0 wz--n- <73.75g    0

The above output indicates that no space is available within the volume group. Suppose, however, that we have a requirement to add 8 GB to the swap on the system. Clearly, this will require the addition of more space to the volume group. For the purposes of this example it will be assumed that a disk that is 8 GB in size and represented by /dev/sdb is available for addition to the volume group. The first step is to turn this partition into a physical volume:

# pvcreate /dev/sdb
  Physical volume "/dev/sdb" successfully created.

If the creation fails with a message similar to “Device /dev/sdb excluded by a filter”, it may be necessary to wipe the disk before creating the physical volume:

# wipefs -a /dev/sdb
/dev/sdb: 8 bytes were erased at offset 0x00000200 (gpt): 45 46 49 20 50 41 52 54
/dev/sdb: 8 bytes were erased at offset 0x1fffffe00 (gpt): 45 46 49 20 50 41 52 54
/dev/sdb: 2 bytes were erased at offset 0x000001fe (PMBR): 55 aa
/dev/sdb: calling ioctl to re-read partition table: Success

Next, the volume group needs to be extended to use this additional physical volume:

# vgextend vgubuntu /dev/sdb
  Volume group "vgubuntu" successfully extended

At this point the vgs command should report the addition of the 10 GB of space to the volume group:

# vgs
  VG        #PV #LV #SN Attr   VSize  VFree  
  vgubuntu   2   2   0 wz--n- 83.74g <10.00g

Now that the additional space is available in the volume group, the swap logical volume may be extended to utilize the space. First, turn off the swap using the swapoff utility:

# swapoff /dev/vgubuntu/swap_1

Next, extend the logical volume to use the new space:

# lvextend -L+9.7GB /dev/vgubuntu/swap_1
  Rounding size to boundary between physical extents: 9.70 GiB.
  Size of logical volume vgubuntu/swap_1 changed from 980.00 MiB (245 extents) to 10.66 GiB (2729 extents).
  Logical volume vgubuntu/swap_1 successfully resized.

Re-create the swap on the logical volume:

# mkswap /dev/vgubuntu/swap_1
mkswap: /dev/vgubuntu/swap_1: warning: wiping old swap signature.
Setting up swapspace version 1, size = 10.7 GiB (11446251520 bytes)
no label, UUID=447fb9e5-5473-4f2c-96f8-839b1457d3ed

Next, turn swap back on:

# swapon /dev/vgubuntu/swap_1

Finally, use the swapon command to verify the addition of the swap space to the system:

# swapon
/dev/dm-1 partition 10.7G   0B   -2

1.8  Summary

Swap space is a vital component of just about any operating system in terms of handling situations where memory resources become constrained. By swapping out areas of memory to disk, the system is able to continue to function and meet the needs of the processes and applications running on it.

Ubuntu has a set of guidelines recommending the amount of disk-based swap space that should be allocated depending on the amount of RAM installed in the system. In situations where these recommendations prove to be insufficient, additional swap space can be added to the system, typically without the need to reboot. As outlined in this chapter, swap space can be added in the form of a file, disk or disk partition or by extending existing logical volumes that have been configured as swap space.