emergency vs rescue systemd targets

[chrismurphy]

In systemd-devel I found some discussion on trying to clarify the difference between them but I'm confused.

Passing the "single" kernel parameter at boot drops me into the rescue.target mode, which makes active a number of targets: basic system, bluetooth, sound, encrypted volumes, local filesystems, sockets and swap. In particular with local filesystems, it seems like I should boot to the emergency.target before running fsck. So I'm not really understanding why single translates into rescue.target. Comments?

[DBelton]

The emergency.target has no corresponding sysvinit runlevel, and would just boot your machine to a shell with really nothing started.

rescue.target is like the old single user or runlevel 1 from sysvinit. It

Here is a short explanation from the developer:

In systemd, "emergency" is little more than an equivalent to
init=/bin/sh on the kernel command like. i.e. you get a shell, but
almost nothing else (except for systemd in the background which you can
then ask to do more). No services are started, no mount points mounted,
no sockets established, nothing. Just a raw, delicious shell and
systemd's promise to be around if you need more. In contrast to that
"rescue" is equivalent to the old runlevel 1 (or S), i.e. sysinit is
run, everything is mounted, but no normal services are started yet.

I think emergency mode is kinda nice for debugging purposes, since it
allows you to boot bit-by-bit, simply by starting in the emergency mode
and then starting the various services and other units that are part of
the early boot step-by-step. This will become particularly useful when
Fedora splits up sysinit into various smaller scripts which could then
be started seperately and independently.

Consider it a part of our boot-up debugging tools.

Lennart