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libc and init system selection

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Systemd

SysVinit and Systemd are common init systems. SysVinit is the default, but Systemd can be enabled be adding the following to site.conf:

DISTRO_FEATURES_append = " systemd"
VIRTUAL-RUNTIME_init_manager = "systemd"
DISTRO_FEATURES_BACKFILL_CONSIDERED = "sysvinit"
VIRTUAL-RUNTIME_initscripts = ""

There is a significant size cost with systemd, so in some cases it may make sense to use sysvinit if image size is a priority.

Busybox Init

Busybox can also be used as an init system with the following in site.conf:

VIRTUAL-RUNTIME_init_manager = "busybox"
VIRTUAL-RUNTIME_dev_manager = "busybox-mdev"
VIRTUAL-RUNTIME_login_manager = "busybox"

Libc selection

glibc is the default libc, but musl can also be used by setting the following in site.conf:

TCLIBC = "musl"

Comparison of disk spaced used

  • Musl + Busybox init/dev/login
    • space used in ext4 filesystem on running system using df: 1.9MB
    • adding sizes of files in image from buildhistory: 1.5MB
    • number of files in image: 595
  • Busybox init/dev/login
    • space used in ext4 filesystem on running system using df: 3.5MB
    • adding sizes of files in image from buildhistory: 3.1MB
    • number of files in image: 621
  • SysVinit
    • space used in ext4 filesystem on running system using df: 4.7MB
    • adding sizes of files in image from buildhistory: 4.3MB
    • number of files in image: 696
  • Systemd
    • space used in ext4 filesystem on running system using df: 33.2MB
    • adding sizes of files in image from buildhistory: 22MB
    • number of files in image: 1,806

The space on disc used by a systemd image is much larger than adding the size of the files in the image. We're not sure why this is -- perhaps there is filesystem overhead for small files.