Add swapfile guide (#28)
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title: How to create (or remove) a swapfile on Linux
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date: 2023-04-04T17:26:44-05:00
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lastmod: 2023-04-04T17:26:44-05:00
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toc: true
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comments: true
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authors:
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- bbaovanc
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categories:
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- guides
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- linux
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- tips-and-tricks
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tags:
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- linux-swap
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- swapfile
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---
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This is a step-by-step tutorial on how to create or remove a swapfile on a Linux
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machine, and whether you should create swap space in the first place.
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Also included is the most correct `fstab` entry: most articles get this "wrong"
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(but it doesn't actually matter).
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<!--more-->
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## Should you use swap?
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A common misconception is that adding swap to your system can reduce
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performance. However, even if you aren't running out of RAM, it can still be
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beneficial to add swap.
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See these points according to [an article by Hayden James][always-add-swap]:
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> - Even if there is still available RAM, the Linux Kernel will **move memory
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> pages that are hardly ever used** into swap space.
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>
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> - It’s better to swap out memory pages that have been inactive for a while,
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> **keeping often-used data in cache**, and this should happen when the server
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> is most idle, which is the aim of the Kernel.
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>
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> - Avoid setting your swap space too large if it will result in prolonging
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> performance issues, outages, or your response time (without proper
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> monitoring/alerts).
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[always-add-swap]: https://haydenjames.io/linux-performance-almost-always-add-swap-space/
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### Should you use a swap file or partition?
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Swap partitions should be preferred because swapfiles tend to be slower and
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slightly more complex to set up, especially if hibernating. A swapfile might be
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preferred due to its flexibility (ease to resize), but if you use
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[LVM][lvm-archwiki] then you can easily resize the swap partition anyways.
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[lvm-archwiki]: https://wiki.archlinux.org/title/LVM
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## Tutorial
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{{< include path="include/bashsession.md" markdown=true >}}
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### Step 1: Create the file
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The first step is to allocate the file.
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```bashsession
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# dd if=/dev/zero of=/swapfile bs=1M count=[size in MiB] status=progress
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```
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Replace `[size in MiB]` with the size of your swapfile in Mebibytes (MiB). You
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can use [this online converter](https://www.convertunits.com/from/GiB/to/MiB) to
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convert from Gibibytes (GiB, often confused with Gigabytes, but that's a
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misconception for another time) to MiB, which you can put in the command.
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Or you can look at this table for common sizes:
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{{< table >}}
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| GiB | MiB |
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|-----|-------------|
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| 1 | count=1024 |
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| 2 | count=2048 |
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| 3 | count=3072 |
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| 4 | count=4096 |
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| 8 | count=8192 |
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| 16 | count=16384 |
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{{< /table >}}
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If you aren't sure how big your swapfile should be, take a look at [Table 15.1
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on this Red Hat documentation page][redhat-swap-table]. Remember that if your
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first swapfile isn't large enough, you can create another one.
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[redhat-swap-table]: https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/storage_administration_guide/ch-swapspace#tb-recommended-system-swap-space
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On my computer I have 32 GiB of RAM and a 16 GiB swap partition, rather than
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swapfile, because I can flexibly expand the swap partition using
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[LVM][lvm-archwiki].
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{{< aside example >}}
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To create a swapfile 4 GiB in size, you would run:
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```bashsession
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# dd if=/dev/zero of=/swapfile bs=1M count=4096 status=progress
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4096+0 records in
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4096+0 records out
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4294967296 bytes (4.3 GB, 4.0 GiB) copied, 0.960183 s, 4.5 GB/s
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```
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{{< /aside >}}
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### Step 2: Change swapfile permissions
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The swapfile should only be readable by the system (`root` user). Run this
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command to change it:
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```bashsession
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# chmod 600 /swapfile
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```
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### Step 3: Format the swapfile
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Use the `mkswap` command to format the file to be used as swap (basically just
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add a header to identify it):
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```bashsession
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# mkswap /swapfile
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Setting up swapspace version 1, size = 4 GiB (4294963200 bytes)
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no label, UUID=a0b87eca-b951-4344-be2d-020d77cdef48
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```
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### Step 4: Create an entry in `/etc/fstab`
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An entry needs to be added to `/etc/fstab` for the swapfile to be enabled during
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bootup. Open `/etc/fstab` in a text editor, and add this line to the end (add
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spaces to line up with other entries if you want, whitespace is ignored):
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```text
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/swapfile none swap sw 0 0
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```
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{{< aside note >}}
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Above, I have filled the "options" field with `sw`. Some guides suggest using
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either `sw` or `defaults`. Neither of those options is valid for `swapon` and
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they are both ignored. However, the field does need to be filled out with
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something, so feel free to put something funny (do let me know in the comments
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if this somehow breaks something though).
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If you're curious, this is my `fstab` entry (`UUID` is because I use a swap
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partition, rather than swapfile):
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```text
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# /dev/mapper/bobavg0-swap
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UUID=4f7c3ae8-839b-4474-b8a5-96bd78db06f8 none swap bobaswap 0 0
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```
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{{< /aside >}}
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### Step 5: Enable the swapfile
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Adding the `fstab` entry won't enable the swapfile until a reboot. To enable it
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now, use the `swapon` command.
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```bashsession
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# swapon /swapfile
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```
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### Finally: check the swap status
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Use `swapon` and `free` to verify that your new swapfile has been added:
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```bashsession
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$ swapon --show
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NAME TYPE SIZE USED PRIO
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/swapfile file 4G 0B -2
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$ free -h
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total used free shared buff/cache available
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Mem: 31Gi 3.6Gi 20Gi 233Mi 6.9Gi 26Gi
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Swap: 4.0Gi 0B 4.0Gi
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```
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{{< aside note >}}
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If you get an error saying that the `swapon` command was not found, try running
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it as `root` (using `sudo`). On Debian-based distributions the `swapon` command
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is not available to regular users.
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```bashsession
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$ swapon --show
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bash: swapon: command not found
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# swapon --show
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NAME TYPE SIZE USED PRIO
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/swapfile file 4G 0B -2
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```
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{{< /aside >}}
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## Removing a swapfile
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{{< aside warning >}}
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Be careful that the swapfile isn't being highly used because once you run
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`swapoff`, it will dump the entire contents back into RAM and may cause you to
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run out of memory.
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{{< /aside >}}
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### Step 1: Disable/unload the swapfile
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```bashsession
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$ swapon --show
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NAME TYPE SIZE USED PRIO
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/swapfile file 4G 0B -2
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$ # the path to the swapfile I want to remove is `/swapfile`
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# swapoff /swapfile
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```
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### Step 2: Remove the entry from `fstab`
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Open `/etc/fstab` in a text editor and find the line matching the swapfile you
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want to remove, and delete it. For example:
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```text
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/swapfile none swap sw 0 0
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```
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### Step 3: Delete the actual file
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```bashsession
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# rm /swapfile
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```
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## References
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### Use `dd` instead of `fallocate`
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See ["Files with holes" under the man page][swapon-holes-man].
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[swapon-holes-man]: https://man.archlinux.org/man/core/util-linux/swapon.8.en#Files_with_holes
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### The options field in fstab
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On Linux, `sw` doesn't mean anything for `swapon`, so it gets ignored. See:
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- [StackExchange answer 1](https://unix.stackexchange.com/a/365961/525130)
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- [StackExchange answer 2](https://unix.stackexchange.com/a/365954/525130)
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- [`parse_options` in the `swapon` source code](https://github.com/util-linux/util-linux/blob/2ea397239683270a0fc8cd3b72ed5457f52dbda8/sys-utils/swapon.c#L699)
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- [options set by fstab `defaults` (none have any effect on `swapon`)](https://man.archlinux.org/man/fstab.5#The_fourth_field_(%3Ci%3Efs_mntops%3C/i%3E).)
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---
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title: Swap (Linux)
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---
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Swap space is storage on your drive that can be used as virtual memory when the
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system is running low on physical memory (RAM). It is also used when hibernating
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(suspend to disk).
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More information: https://haydenjames.io/linux-performance-almost-always-add-swap-space/
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<!--more-->
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@ -0,0 +1,11 @@
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---
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title: swapfile
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---
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A swapfile is a file that gives your system more [swap]({{< relref
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"../linux-swap" >}}) space, which is used when the system is running low on RAM,
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or when hibernating.
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<!--more-->
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@ -1,4 +1,4 @@
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{{< aside tip >}}
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{{< aside info >}}
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Commands in this article are prefixed them with a prompt symbol (either `$` or
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`#`) which is not part of the command. Lines with no prompt symbol are output
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@ -17,6 +17,8 @@ $ # regular users aren't allowed to view /var/lib/docker, we need root
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builder containerd image overlay2 runtimes tmp volumes
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buildkit containers network plugins swarm trust
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$ # we can do the same thing using sudo
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$ sudo ls /var/lib/docker
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[sudo] password for bbaovanc:
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builder containerd image overlay2 runtimes tmp volumes
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