So I'm back on systemd now, mainly because I really like volatile system logs with journald.
Following the article in the Gentoo Wiki, things went mostly smooth - much better than with my first attempt during installation. However, there were two little pitfalls concerning the LUKS volume my rootfs resides on.
Here's my partitioning layout:
sdb 8:16 0 119,2G 0 disk
├─sdb1 8:17 0 512M 0 part /boot
└─sdb2 8:18 0 118,8G 0 part
└─luksroot 254:0 0 118,8G 0 crypt
├─m4vg0-swap 254:1 0 512M 0 lvm [SWAP]
├─m4vg0-root 254:2 0 30G 0 lvm /
└─m4vg0-home 254:3 0 88,2G 0 lvm /home
In order to get the LVM PV unlocked, I originally had the following in my kernel cmdline passed to dracut:
(...) crypt_root=UUID=2a153448-c20e-46ea-a5f9-148fcb71a907 root_trim=yes rootfstype=ext4 (...)
With systemd inside the initramfs generated by dracut the syntax is a bit different. I initially changed it to
(...) rd.luks.uuid=2a153448-c20e-46ea-a5f9-148fcb71a907 rd.luks.allow-discards rootfstype=ext4 (...)
and this basically worked but disabled TRIM (see this bug) and yielded a warning:
dracut-initqueue[295]: Warning: Unit file of systemd-cryptsetup@luks\x2d2a153448\x2dc20e\x2d46ea\x2da5f9\x2d148fcb71a907.service changed on disk, 'systemctl daemon-reload' recommended.
The fix for both was to put an entry for the LUKS volume in
/etc/crypttab and set the discard option there:
# /etc/crypttab
luksroot UUID=2a153448-c20e-46ea-a5f9-148fcb71a907 none discard
(...)
The corresponding entries from the kernel cmdline can then be omitted, as the crypttab is included in initramfs and automatically parsed.
Update 2015-05-25:
As Lennart Poetering says here,
the often mentioned option rd.luks.allow-discards is currently not
implemented and one should use rd.luks.options=discard instead.
This avoids the aforementioned entry in /etc/crypttab, when desired for all volumes.
Credits for this update go to Nils Freydank - thanks! :)