[systemd-devel] Question about generators and adding new units in the middle of a transaction

[Mirco Tischler]

2011/11/4 Manuel Amador <rudd-o at rudd-o.com>:
> I am developing systemd support for ZFS:
>
> https://github.com/zfsonlinux/zfs/pull/435/files
>
> as you can see, I create the units early on bootup using a generator (a
> mechanism that is entirely undocumented, tsk).
>
> Then systemd proceeds with normal system startup.
>
> The whole point is to be able to mount file systems of other types on top of
> ZFS file systems, and then ZFS file systems on top of that.  This work lets
> this scenario work properly:
>
> / zfs
> /blah ext4
> /blah/blahblah zfs
>
> But, here is a problem.  This works fine and dandy when ZFS has loaded the
> pools at boot through dracut or something, but will most assuredly fail if ZFS
> is not the root file system, as nothing will load the ZFS module.
>
> We have some udev mechanisms at the moment to ensure that actually happens
> (loading of the zfs modules, importing of all pools).
>
> Good and dandy so far.
>
> Now, this will happen during udev settle.  What I want is to generate more
> units when pools are discovered and their file systems require to be mounted
> automatically.  That is, I need to re-run the generator and generate new
> units, and then tell systemd to daemon-reload.
>
> But systemd is in the middle of a transaction, serving the unit local-
> fs.target.  And, as you can imagine, the file systems that were discovered
> late, must be linked as wants of local-fs.target.
>
> So my question is: what happens if I systemctl daemon-reload DURING the
> transaction that brings the system up?  Will systemd pick up the new units and
> add them as wants of local-fs.target?
>
> ideal process:
>
> root fs is mounted
> starting local-fs.target
> starting block device discovery
> block dev discovered, import pool in block dev
> oh, we found new file systems!
> generate units for those
> daemon-reload to add the new units as wants for local-fs.target
> start all of these new units
> and then, only then, local-fs.target will reach started state.
>
> Is this even possible??

Hi
I know very little about ZFS so please excuse my ignorance, but I
don't understand the problem you are trying to solve. Systemd parses
/etc/fstab already and creates the mount units. And the necessary
modules should be loaded automatically on mount, just like with all
the other filesystems. And udev works nicely with systemd to announce
new block devs. What differs in ZFS that this isn't working for you?

Thanks
Mirco