How to Limit Tmpfs for Taking Swap Space

Introduction

A tmpfs filesystem is a temporary, memory-based filesystem that resides in virtual memory, providing extremely fast file access. It dynamically allocates RAM and can use swap space when needed, consuming only as much memory as required for its contents. However, its contents are volatile, meaning all data is lost when the filesystem is unmounted. See tmpfs(5) for more details.

Problem

The user doesn't want tmpfs to use swap space. For example, swap space set aside for slurm jobs.

Resolution

As mentioned in the warewulf documentation, the wwinit module provisions to tmpfs. By default, tmpfs is permitted to use up to 50% of physical memory. This size limit may be adjusted using the kernel argument wwinit.tmpfs.size. (This parameter is passed to the size option during tmpfs mount.)

wwinit.tmpfs.size is only used by dracut. To boot with dracut please follow this guide..

If you aren’t using dracut this parameter won’t adjust the tmpfs limit, and is not currently configurable, aside from manually editing the init file in the overlay, or maintaining a copy of it locally.

tmpfs blocks may be swapped out, when there is a shortage of memory. tmpfs has a mount option to disable its use of swap:

noswap - # Disables swap. Remounts must respect the original settings. By default swap is enabled.

This can also be managed in the init file

Notes

Kernel args can still be set with dracut boot. However, --kerneloverride as currently implemented would require the ability to push the matching kernel modules into the dracut image, which is not currently supported.

References & related articles

Warewulf - Booting with Dracut
Man tmpfs page
Man dracut page