Introducing swap space concepts
A swap space is an area of a disk under the control of the Linux kernel memory management subsystem. The kernel uses swap space to supplement the system RAM by holding inactive pages of memory. The combined system RAM plus swap space is called virtual memory.
When the memory usage on a system exceeds a defined limit, the kernel searches through RAM looking for idle memory pages assigned to processes. The kernel writes the idle pages to the swap area and reassigns the RAM pages to other processes. If a program requires access to a page on disk, the kernel locates another idle page of memory, writes it to disk, then recalls the needed page from the swap area.
Because swap areas reside on disk, swap is slow when compared with RAM. While it is used to augment system RAM, you should not consider swap space as a sustainable solution for insufficient RAM for your workload.
Sizing the Swap Space
Administrators should size the swap space based on the memory workload on the system. Application vendors sometimes provide recommendations on that subject. The following table provides some guidance based on the total amount of physical memory.
RAM and Swap Space Recommendations
|RAM||SWAP SPACE||SWAP SPACE IF ALLOWING FOR HIBERNATION|
|2 GiB or less||Twice the RAM||Three times the RAM|
|Between 2 GiB and 8 GiB||Same as RAM||Twice the RAM|
|Between 8 GiB and 64 GiB||At least 4 GiB||1.5 times the RAM|
|More than 64 GiB||At least 4 GiB||Hibernation is not recommended|
The laptop and desktop hibernation function uses the swap space to save the RAM contents before powering off the system. When you turn the system back on, the kernel restores the RAM contents from the swap space and does not need a complete boot. For those systems, the swap space needs to be greater than the amount of RAM. The Knowledgebase article in the Reference section at the end of this section gives more guidance on sizing the swap space.
Creating a swap space
To create a swap space, you need to perform the following:
- Create a partition with a file system type of linux-swap.
- Place a swap signature on the device.
Creating a Swap Partition
Use parted to create a partition of the desired size and set its file system type to linux-swap. In the past, tools looked at the partition file system type to determine if the device should be activated; however, that is no longer the case. Even though utilities no longer use the partition file system type, setting that type allows administrators to quickly determine the partition’s purpose.
The following example creates a 256 MB partition.
[root@host ~]# parted /dev/vdb GNU Parted 3.2 Using /dev/vdb Welcome to GNU Parted! Type 'help' to view a list of commands. (parted) print Model: Virtio Block Device (virtblk) Disk /dev/vdb: 5369MB Sector size (logical/physical): 512B/512B Partition Table: gpt Disk Flags: Number Start End Size File system Name Flags 1 1049kB 1001MB 1000MB data (parted) mkpart Partition name? ? swap1 File system type? [ext2]? linux-swap Start? 1001MB End? 1257MB (parted) print Model: Virtio Block Device (virtblk) Disk /dev/vdb: 5369MB Sector size (logical/physical): 512B/512B Partition Table: gpt Disk Flags: Number Start End Size File system Name Flags 1 1049kB 1001MB 1000MB data 2 1001MB 1257MB 256MB linux-swap(v1) swap1 (parted) quit Information: You may need to update /etc/fstab.
After creating the partition, run the udevadm settle command. This command waits for the system to detect the new partition and to create the associated device file in /dev. It only returns when it is done.
[root@host ~]# udevadm settle [root@host ~]#
Formatting the Device
The mkswap command applies a swap signature to the device. Unlike other formatting utilities, mkswap writes a single block of data at the beginning of the device, leaving the rest of the device unformatted so the kernel can use it for storing memory pages.
[root@host ~]# mkswap /dev/vdb2 Setting up swapspace version 1, size = 244 MiB (255848448 bytes) no label, UUID=39e2667a-9458-42fe-9665-c5c854605881
Activating a swap space
You can use the swapon command to activate a formatted swap space. Use swapon with the device as a parameter, or use swapon -a to activate all the swap spaces listed in the /etc/fstab file. Use the swapon –show and free commands to inspect the available swap spaces.
[root@host ~]# free total used free shared buff/cache available Mem: 1873036 134688 1536436 16748 201912 1576044 Swap: 0 0 0
[root@host ~]# swapon /dev/vdb2
[root@host ~]# free total used free shared buff/cache available Mem: 1873036 135044 1536040 16748 201952 1575680 Swap: 249852 0 249852
You can deactivate a swap space using the swapoff command. If the swap space has pages written to it, swapoff tries to move those pages to other active swap spaces or back into memory. If it cannot write data to other places, the swapoff command fails with an error, and the swap space stays active.
Activating Swap Space Persistently
To activate a swap space at every boot, place an entry in the /etc/fstab file. The example below shows a typical line in /etc/fstab based on the swap space created above.
UUID=39e2667a-9458-42fe-9665-c5c854605881 swap swap defaults 0 0
The example uses the UUID as the first field. When you format the device, the mkswap command displays that UUID. If you lost the output of mkswap, use the lsblk –fs command. As an alternative, you can also use the device name in the first field.
- The second field is typically reserved for the mount point. However, for swap devices, which are not accessible through the directory structure, this field takes the placeholder value swap.
- The third field is the file system type. The file system type for swap space is swap.
- The fourth field is for options. The example uses the defaults option. The defaults option includes the mount option auto, which means activate the swap space automatically at system boot.
- The final two fields are the dump flag and fsck order. Swap spaces require neither backing up nor file-system checking and so these fields should be set to zero.
When you add or remove an entry in the /etc/fstab file, run the systemctl daemon-reload command, or reboot the server, for systemd to register the new configuration.
[root@host ~]# systemctl daemon-reload
Setting the Swap Space Priority
By default, the system uses swap spaces in series, meaning that the kernel uses the first activated swap space until it is full, then it starts using the second swap space. However, you can define a priority for each swap space to force that order.
To set the priority, use the pri option in /etc/fstab. The kernel uses the swap space with the highest priority first. The default priority is -2.
The following example shows three swap spaces defined in /etc/fstab. The kernel uses the last entry first, with pri=10. When that space is full, it uses the second entry, with pri=4. Finally, it uses the first entry, which has a default priority of -2.
UUID=af30cbb0-3866-466a-825a-58889a49ef33 swap swap defaults 0 0 UUID=39e2667a-9458-42fe-9665-c5c854605881 swap swap pri=4 0 0 UUID=fbd7fa60-b781-44a8-961b-37ac3ef572bf swap swap pri=10 0 0
Use swapon –show to display the swap space priorities. When swap spaces have the same priority, the kernel writes to them in a round-robin fashion.