Re: ZFS on Debian GNU/kFreeBSD

[Seth Goldberg]

Quoting Robert Millan, who wrote the following on Fri, 23 Jul 2010:

> The problem I see with current zfs.mod is that it integrates with GRUB using
> the "filesystem model", i.e. each pool has one
> backend (the storage device) and one frontend (the filesystem).
>
> But ZFS is different in both ends: each pool can use N storage devices
> and provide M filesystems. In the current representation this yields
> two problems
> for GRUB:
>
> - zpools where more than one device is essential (i.e. not just
> mirror like RAID1)
>   aren't accessible.

  Very true, and if non-mirrored root pools are supported in the future, that 
will be a problem.

> - To allow more more than one filesystem in a zpool, filesystems are
> selected in
>   a special manner, e.g.:
>   (hd0)/@/
>   (hd0)/foo@/
>   This collides with an assumption consistently made by GRUB utilities
>   and scripts: that you can construct a GRUB path by combining the result
>   of make_path_relative_to_its_root() (known at install time) with the device
>   that contains this filesystem (usually known at run time).

  Well, that still does work -- you just need to modify 
make_path_relative_to_its_root ;).


> My impression is representing ZFS as a filesystem is doable, but not
> the best fit.
> ZFS is much more like what GRUB calls an "abstraction" (RAID or LVM). In fact
> if you take LVM interface:
>
> grub> ls
> (hd0) (hd1)
> grub> insmod lvm
> grub> ls
> (hd0) (hd1) (foo) (bar)
>
> the only difference for ZFS is that (foo) and (bar) are provided directly as
> filesystems (like pxe.mod does) instead of block devices for GRUB to probe.

  How do you deal with multiple pools each with the same filesystem names?  We 
do need a namespace -- top-level should be (<poolname>) (though keep in mind 
it's possible for different pools with the same pool name to exist, so we'd 
need a uniquifier here also).

> So my intention is to readjust zfs.mod to follow this interface. Aside from 
> enabling multi-device pools, this will also make it much easier to support 
> ZFS in the install utilities and scripts later.
>
> Comments / objections / suggestions?

  Sounds like an interesting experiment, at least :).

 --S