systemd-fsckd.service is a service responsible for receiving file system check progress, and communicating some consolidated data to console and plymouth (if running). It also handles possible check cancellations.
systemd-fsckd receives messages about file system check progress from fsck through an UNIX domain socket. It can display the progress of the least advanced fsck as well as the total number of devices being checked in parallel to the console. It will also send progress messages to plymouth. Both the raw data and translated messages are sent, so compiled plymouth themes can use the raw data to display custom messages, and scripted themes, not supporting i18n, can display the translated versions.
systemd-fsckd will instruct plymouth to grab Control+C keypresses. When the key is pressed, running checks will be terminated. It will also cancel any newly connected fsck instances for the lifetime of systemd-fsckd.
systemd-fsckd passes the following messages to the theme:
Progress update, sent as a plymouth update message: "fsckd:<num_devices>:<progress>:<string>"
Cancel message, sent as a traditional plymouth message: "fsckd-cancel-msg:<string>"
The following options are understood: