Create and Enable Swap¶

What is swap and why use it?¶

Swap extends available memory by moving inactive pages from RAM to a file or partition on storage.
On FRITZ!Box devices with limited RAM, swap can improve stability when running memory-heavy packages.

Important: swap on flash/USB storage is slower than RAM and increases write activity.
Use moderate sizes and avoid over-relying on swap for performance.

Prerequisite: enable Swap options in build config¶

If Settings -> Swap is missing in the WebIF, enable it in build configuration:

Run make menuconfig
Open Additional patches
Enable Add swap options (FREETZ_ADD_SWAPOPTIONS)
Rebuild and flash firmware

After flashing, open:

http://fritz.box:81/cgi-bin/conf/mod

Method A: create swap from WebIF¶

In Settings -> Swap:

Set Path to a writable storage location, for example:
/var/media/ftp/uStor01/swapfile
or a partition path such as /dev/sda1
Set Size in MB (when creating a swap file)
Click Create swap file
Set start mode to Automatic
Save settings

To activate immediately without reboot:

Open Services
Start service Swap

Method B: create swap file manually (Linux shell)¶

Use this if you prefer shell-based setup or are on older branches.

Example (64 MB):

Bash

dd if=/dev/zero of=swapfile bs=1k count=64000
mkswap swapfile
scp swapfile root@fritz.box:/var/media/ftp/uStor01/

Then configure the same path in Settings -> Swap, enable Automatic, save, and start the Swap service.

Verification¶

From shell, check active swap:

Bash

cat /proc/swaps

Expected result: your configured swap file or partition appears in the list.

Recommended values¶

Typical size for routers: 64 to 256 MB
swappiness: start with 60, then tune if needed
Prefer USB/external storage over internal flash when possible

Troubleshooting¶

Swap section not visible in WebIF: enable FREETZ_ADD_SWAPOPTIONS and rebuild.
Service fails to start: verify file path exists and permissions allow access.
No entry in /proc/swaps: check service status and run start manually from Services page.