feature runtime compressed fs

Feature: Compressed Initramfs

Overview

This feature introduces support for building a compressed initramfs to significantly reduce memory usage during system boot, especially in memory-constrained environments. By consolidating most files into a compressed, read-only squashfs image, the number of files loaded directly into ramfs is minimized, reducing both memory consumption and page fragmentation.

Two backends are supported:

• squashfs is a compressed read-only filesystem for linux (https://docs.kernel.org/filesystems/squashfs.html);
• erofs - Enhanced Read-Only File System (https://erofs.docs.kernel.org/en/latest/index.html).

Background

• Problem: Traditional initramfs stores all files in ramfs, which is uncompressed and allocates at least one page per file, regardless of file size. This leads to:

  • High memory usage, especially with many small files.
  • Increased waste on systems with large page sizes due to page fragmentation.

• Solution: Move most files into a compressed squashfs/erofs image, keeping only a minimal set of executables and libraries outside as a loader. This reduces the number of files in ramfs and compresses their contents.

Parameters

• RUNTIME_BASE_LAYER -- If not empty, it determines the type of backend image. Maybe squashfs or erofs. If empty, it is detected by the existence of utilities.
• RUNTIME_LEYER_SQUASHFS_COMPRESS -- The parameter specifies a compression algorithm (default: xz).
• RUNTIME_LAYER_SQUASHFS_ARGS -- Extra arguments for mksquashfs.
• RUNTIME_LEYER_EFOFS_COMPRESS -- The parameter specifies a compression algorithm.
• RUNTIME_LEYER_EFOFS_ARGS -- Extra arguments for mkfs.erofs.