RootEphemeral=, which lets processes run in a filesystem snapshot; super neat for seeing what changes a process might make to your system. https://github.com/binpash/try was a popular submission a couple days ago, & this builds in a similar capability to the os.
RestartSteps= and RestartMaxDelaySec= to finally finally offer some level of control over exponential-backoff in the restart behavior.
FileDescriptorStorePreserve= is kind of mysterious, lets systemd store open file descriptors for a process even after it terminates. Some interaction with systemd-analyze. Not sure what this is for exactly but definitely cool extra visibility/observability of processes. I tend to think it also potentially could be useful allow for some weird/fun stuff like restarting services without closing ports.
Soft-reboot which can be useful for switching roots without having to juggle bootloading. Very useful for anyone doing partitioning or what-not. And as I hinted above, you can pass file-descriptors, meaning you can avoid service-interruption while rebooting; what a neat trick.
DelegateSubgroup= puts processes inside a sub-cgroup inside the top level cgroup. So as a user you could have a bunch of sub-cgroups that you can manage, letting you juggle/manage resources within your cgroup. This sounds a lot like Dixon's incredible talk technique of cgroup management, Linux memory management at scale. https://www.youtube.com/watch?v=beefUhRH5lU&t=2241s This is also easily what I think the worst most pitiful saddest part of the Kubernetes story is: a complete unwillingness to let Linux manage memory & use cgroups well, in favor of it's own scheduler which makes all the decisions.
iocost calculations for io devices, to configure/tune io/qos cost for devices. has a huge existing db of drives. https://github.com/iocost-benchmark/iocost-benchmarks
systemd-sysext gains a systemd-confext twin. the former manages overlays for /usr and /opt, the latter manages overlays for /etc. Very nice portable services tool now. Maybe the capstone work completing Revisiting how we put together Linux Systems. https://0pointer.net/blog/revisiting-how-we-put-together-lin... .
Using efi vars to save the hibernation partition, which should be a huge win for hibernation working everywhere.
Some other neat bits:
CAP_WAKE_ALARM rights to user sessions, for setting long-running timers. Sigqueue support to send processes signals with associate values. list-paths verb to show paths. systemd.tty.* kernel arguments for setting up ttys. Early load virtio_console module if it detects it's runnin in a VM to get console up earlier. Upholds= gets a new .upholds/ drop-in directory for keeping other services running (not just starting them like Wants=). sd_journal_get_seqnum api call so apps can have mechanistic sympathy with journald. Ton of bootloader, boot-security, repartitioning, disk-encryption enhancements across the map. SetTTY to for updating the TTY of a session; so useful for ssh'ing in.
udev creates /dev/loop/by-ref/* files with helpful names. systemd-resolved gets a StateRetentionSec= to let you cache & use old records if a nameserver isn't responding. Most systemd services now have some standard signals they accept to tweak config, ex: changing log level, log target, trim memory, print debug mem info. machinectl & networkctl gain "edit" and "cat" capabilities for viewing config. systemd-firstboot --reset which is hella useful for cloning machines & updating machine-id. Services now sd_notify with their EXIT_STATUS on exit; part of trend of making services more observable/operable everywhere. new systemd.mount-extra= kernel arg to ask for mounts at startup.
I love reading release notes in general, but this was absurdly delightful. It felt like in the past sometimes systemd was focusing on new feature growth & ideas, but I'm seeing huge maturation & growing in in this release. Really leaning hard into everything alive & responsive on via signals and passing around file-descriptors is excellent excellent excellent systems-wonkery. The fit and finish here is going way up. There's just a ton of damned-useful ergonomic wins that make everything super easy to find & see & touch & manage. 254 is an epic release.