[systemd-devel] systemd-fsck-root semantics

[Zbigniew Jędrzejewski-Szmek]

On Thu, Jul 03, 2014 at 11:49:20AM +0200, Thomas Bächler wrote:
> Am 02.07.2014 14:29, schrieb Daniel Drake:
> > If I'm reading things right, actually the default behaviour is (when
> > no hints are supplied in kernel cmdline) :
> >  1. systemd runs fsck on root from initramfs
> >  2. systemd mounts root fs ro
> >  3. switch-root onto real system
> >  4. systemd-fsck-root runs
> >  5. systemd-remount-fs remounts / as rw
> > 
> > Also just noticed another interesting thing -
> > systemd-fsck-root.service is only loaded dynamically, when /etc/fstab
> > has a non-zero passno for /. So maybe the idea is that anyone running
> > a regular and modern dracut/systemd setup sets passno=0 for / in
> > fstab, with the knowledge that fsck of / is done by the initramfs.
> 
> Tom and me are thought this through a couple of months ago when I
> started looking into using Arch's mkinitcpio with systemd. We submitted
> a few changes to fsck handling back then.
> 
> This is the setup we want to support (this is now the default in a new
> Arch installation):
> 1) fsck in initramfs
> 2) initramfs mounts rw
> 3) no re-fsck in the system
This is what we want.

We also want this to work:
1) fsck in the initramfs
2) initramfs mounts ro
3) no re-fsck or remount in the system

> This is still supported, for legacy systems that have not been
> reconfigured properly:
> 1) no fsck in initramfs, or no initramfs
> 2) initramfs or kernel mount ro
> 3) fsck in the system
> 4) remount rw in the system
> 
> 
> This setup works, but is not optimal:
> 1) fsck in initramfs
> 2) initramfs mounts ro
> 3) re-fsck in the system
> 4) remount rw in the system
> In this case, I always recommend mounting rw right away. However, you
> can also configure your fstab to skip fsck - I don't see why anyone
> would prefer this though.
> 
> For your read-only case, you have two options:
> 
> * Have an fstab entry for / with passno=0
> * Don't list / in fstab
> 
> I prefer the last option, since there is no benefit in adding / to fstab
> - IMO, the correct mount options should always be given via the kernel
> commandline to make remounting unnecessary.
> 
> In my opinion, there is no need to change anything in systemd here,
> since all the right configuration options are already there.
You're right that everything *can* be configured to work properly.
But if any fs was already checked in the initramfs, systemd should be
always smart enough to avoid checking it again in the main system.
I think that this is works for all filesystems except root. For root
it breaks when the fs is mounted ro when transitioning to main system.

Zbyszek