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.type). These checkers will decide if the filesystem should actually be checked based on the time since last check, number of mounts, unclean unmount, etc.
systemd-fsck-root.service and systemd-fsck-usr.service will activate reboot.target if fsck 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.
Kernel Command Line
systemd-fsck understands these kernel command line parameters:
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.
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.
systemd(1), fsck(8), systemd-quotacheck.service(8), fsck.btrfs(8), fsck.cramfs(8), fsck.ext4(8), fsck.fat(8), fsck.hfsplus(8), fsck.minix(8), fsck.ntfs(8), fsck.xfs(8)
kernel-command-line(7), systemd.directives(7), systemd.index(7), systemd-quotacheck.service(8).
The man pages systemd-fsck(8), systemd-fsck-root.service(8) and systemd-fsck-usr.service(8) are aliases of systemd-fsck@.service(8).