[systemd-devel] [PATCH] fstab-generator: local-fs.target waits for nofail mounts

[Andrey Borzenkov]

В Thu, 10 Apr 2014 09:46:56 -0400
Vivek Goyal <vgoyal at redhat.com> пишет:

> On Thu, Apr 10, 2014 at 06:38:59AM +0400, Andrey Borzenkov wrote:
> 
> [..]
> > > So with nofail opion for rootfs we should have following situation.
> > > 
> > > - sysroot.mount
> > > 	Before=initrd-root-fs.target
> > > - initrd-root-fs.target
> > > 	Requires=sysroot.mount
> > > 	OnFailure=emergency.target
> > > - initrd.target
> > > 	Wants=initrd-root-fs.target
> > > 	OnFailure=emergency.target
> > > - dracut-pre-pivot.service
> > > 	After=initrd.target sysroot.mount
> > > 
> > > Now let us say sysroot.mount failed activation because root device did not
> > > show up. We waited for certain time interval, then time out. Now what will
> > > happen to initrd-root-fs.target and initrd.target states.
> > > 
> > 
> > Assuming initrd-root-fs.target Requires sysroot.mounts it enters Failed
> > state and systemd effectively executes analog of "systemctl start
> > emergency.target". What happens after that is defined entirely by what
> > emergency.target pulls in. 
> > 
> > initrd.target in your example does not depend on sysroot.mount in any
> > way so unless there are further indirect dependencies it actually
> > should be reached at this point.
> 
> initrd.target Wants initrd-root-fs.target which inturn depends on
> sysroot.mount.

Wants does not imply any sort of mandatory dependency. Unit A wants
unit B means that when systemd gets request to start A it will
additionally attempt to start B. Whether it will actually be able to
start B is irrelevant.

>               systemd automatically generates a Requires=sysroot.mount
> in initrd-root-fs.target.
> 
> So if sysroot.mount fails, that should start emergency.target as
> initrd-root-fs.target will fail. 
> 
> As initrd.target has Wants=initrd-root-fs.target, and
> initrd-root-fs.target activation has failed. So does that mean that
> initrd.target will reach the failed state too and we will try to
> launch emergency.target.
> 

No, unless it has some other mandatory dependency (Requires, BindsTo).
As already explained, Wants does not imply any sort of dependency
between unit states; it is simply convenient shortcut to start multiple
units at once. After start job is submitted, units run independently on
each other.

> What will happen to dracut-pre-pivot.service. It is supposed to run
> after intird.target has reached. Now initrd.target has failed activation.
> Will dracut-pre-pivot.service be activated?
> 

Yes. Again, After does not imply any mandatory dependency. A After B
only says that when two units are started together, A should wait until
B startup is complete. If B fails to start, its startup obviously is
complete :) so systemd will just continue.

One thing I'm not sure about - what "are started together" actually
means. I.e. common sense suggests that "systemctl start A B" causes
them both be started together. But what if I do "systemctl start A"
and later "systemctl start B" while job for A is still running and B
has After=A. Will it wait for A startup to complete?