systemd-fsck@.service, systemd-fsck-root.service, systemd-fsck-usr.service, systemd-fsck — File system checker logic
| systemd-fsck@.service | 
| systemd-fsck-root.service | 
| systemd-fsck-usr.service | 
| /usr/lib/systemd/systemd-fsck | 
systemd-fsck@.service, systemd-fsck-root.service, and
    systemd-fsck-usr.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 and systemd-fsck-usr.service are
    responsible for file system checks on the root and /usr file system, respectively, but only if the root
    filesystem was not checked in the initrd. systemd-fsck@.service is used for all
    other file systems and for the root file system in the initrd.
These services are started at boot if passno in /etc/fstab
    for the file system is set to a value greater than zero, but only if it is also configured to be
    mounted at boot (i.e. without "noauto" option). 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 on 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
    (fsck.). These checkers will decide if
    the filesystem should actually be checked based on the time since
    last check, number of mounts, unclean unmount, etc.type
systemd-fsck-root.service and systemd-fsck-usr.service
    will activate reboot.target if
    fsck(8)
    returns the "System
    should reboot" condition, or emergency.target if fsck
    returns the "Filesystem errors left uncorrected" condition.
systemd-fsck@.service will fail if
    fsck returns with either "System should reboot"
    or "Filesystem errors left uncorrected" conditions. For filesystems
    listed in /etc/fstab without "nofail"
    or "noauto" options, "local-fs.target"
    will then activate emergency.target.
systemd-fsck understands these kernel command line parameters:
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.