systemd-fsck@.service, systemd-fsck-root.service, systemd-fsck — File system checker logic
systemd-fsck@.service
systemd-fsck-root.service
/usr/lib/systemd/systemd-fsck
systemd-fsck@.service and
    systemd-fsck-root.service are services
    responsible for file system checks. They are instantiated for each
    device that is configured for file system checking.
    systemd-fsck-root.service is responsible for
    file system checks on the root file system, but in only if the
    root filesystem wasn't checked in the initramfs.
    systemd-fsck@.service is used for all other
    file systems and for the root file system in the initramfs.
Those services are started at boot if
    passno in /etc/fstab for the
    file system is set to a value greater than zero. The file system
    check for root is performed before the other file systems. Other
    file systems may be checked in parallel, except when they are one
    the same rotating disk.
systemd-fsck does not know any details
    about specific filesystems, and simply executes file system
    checkers specific to each filesystem type
    (/sbin/fsck.*). This helper will decide if
    the filesystem should actually be checked based on the time since
    last check, number of mounts, unclean unmount, etc.
systemd-fsck will forward file system
    checking progress to the console. If a file system check fails for
    a service without nofail, emergency mode is
    activated, by isolating to
    emergency.target.
systemd-fsck understands one kernel
    command line parameter:
fsck.mode=¶One of "auto",
        "force", "skip". Controls
        the mode of operation. The default is "auto",
        and ensures that file system checks are done when the file
        system checker deems them necessary. "force"
        unconditionally results in full file system checks.
        "skip" skips any file system
        checks.
fsck.repair=¶One of "preen",
        "yes", "no". Controls the
        mode of operation. The default is " preen",
        and will automatically repair problems that can be safely
        fixed. "yes " will answer yes to all
        questions by fsck and "no" will answer no to
        all questions.