Booting from zfs snapshot

[Christopher J. Ruwe]

On Mon, 23 Jan 2012 14:42:19 +0100
Andreas Nilsson <andrnils at gmail.com> wrote:

> On Mon, Jan 23, 2012 at 1:58 PM, Johannes Totz <johannes at jo-t.de>
> wrote:
> 
> > On 22/01/2012 20:15, Andreas Nilsson wrote:
> >
> >> On Sun, Jan 22, 2012 at 1:25 PM, Johannes Totz<johannes at jo-t.de>
> >> wrote:
> >>
> >>  On 19/01/2012 16:26, Andreas Nilsson wrote:
> >>>
> >>>  Hello,
> >>>>
> >>>> I'm trying to wrap my head around the process of booting from a
> >>>> zfs snapshot. I have hit a few roadblocks, which I hope this is
> >>>> the adequate list to post to regarding those.
> >>>>
> >>>> A short note on what I'm trying to achieve might be in order. In
> >>>> short: a
> >>>> nanobsd system on zfs only. I want to boot from a snapshot so
> >>>> that when I
> >>>> push out an upgrade with zfs send, I want the root filesystem to
> >>>> remain unchanged.
> >>>>
> >>>> The problems I've hit so far:
> >>>> *1 Making the zpool.cache file available
> >>>> *2 Having / mount via entry in fstab.
> >>>>
> >>>>
> >>> FWIW, I dont use any fstab for my zfs-only machine. Works
> >>> perfectly fine with mountpoint property.
> >>>
> >>
> >>
> >> It was just a small hope that the boot would continue with the
> >> filesystem pointed out by the bootfs property. I'm setting
> >> vfs.root.mountfrom in loader.conf now.
> >>
> >>
> >>
> >>>
> >>>  *1: The zpool.cache is needed to autoimport a pool as I
> >>> understand it. Is
> >>>
> >>>> there a way to force the kernel to import a pool during bootup
> >>>> even though
> >>>> no zpool.cache is around? What does this file actually contain?
> >>>>
> >>>> I made an experiment and booted a disk with zfs root from
> >>>> machine a in machine b and that worked. I did partition the disk
> >>>> with gpart using a gpt
> >>>> scheme, and labeled the partition on which the pool resides as
> >>>> os, and upon
> >>>> creation of the zpool used gpt/os as device. Does this mean that
> >>>> as long as
> >>>> gpt/os is available, any machine boot this disk will have the
> >>>> zpool autoimported?
> >>>>
> >>>>
> >>> Not quite sure I understand you here. Just a note: booting kernel
> >>> and mounting root fs are two different things. the *zfsloader
> >>> will happily load
> >>> the kernel off a pool and boot it but mounting root might fail
> >>> later (I guess if no cachefile is present?).
> >>>
> >>>  Sorry for being unclear. What I experience is exactly what you're
> >> describing: gptzfsloader loads the kernel and runs it just fine,
> >> but mounting root fails due to missing zpool.cache.
> >>
> >> I'm looking for a way to have the pool imported without the
> >> zpool.cache
> >>
> >>
> >>
> >>>
> >>>  *2: Having a line like
> >>>> tank/root/8.2-RELEASE-p5 at ro / zfs ro 0 0
> >>>> in fstab causes mount to throw an error and leave me in single
> >>>> user mode,
> >>>> when the system is booted however mount can mount a zfs snapshot
> >>>> just fine.
> >>>> Setting vfs.root.mountfrom in loader.conf works just fine though.
> >>>>
> >>>>
> >>>  The above I still think is rather strange: setting
> >>> vfs.root.mountfrom
> >> to a
> >> snapshot ( given that the snapshot has the zpool.cache file)
> >> works, but not
> >> having the corresponding line in fstab.
> >>
> >>
> >>>
> >>> --
> >>> Sent from my<insert random gadget here>
> >>>
> >>>
> >> What I'm seeking a solution to is this: Boot several machines from
> >> one zfs snapshot. Since the stream from zfs send is also used to
> >> do the initial install there is no easy way to get the correct
> >> zpool.cache file in the snapshot. I guess one possible way to
> >> tackle this problem is to modify zpool so that all pools get
> >> created with the same id.
> >>
> >
> > There's a way to pre-load a zpool.cache via the loader (from an
> > alternative location). Can't remember the correct variables to set
> > right now...
> >
> > As for w/o any cache file... no idea.
> >
> > Ok, I'll research that. If it could be loaded from another
> > disk/partition
> I could really use it :)
> 
>  It is of course possible to edit the cachefile property of the
> zpool, but I think one can only set it to something relative the
> dataset from which one boots.
> 
> Regards
> Andreas
> _______________________________________________
> freebsd-fs at freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-fs
> To unsubscribe, send any mail to "freebsd-fs-unsubscribe at freebsd.org"


Just a curious question: Will what you are trying to implement be
similar to Solaris boot environments/beadm? 

In short, Solaris boot environments are similar to what you described
(booting of snapshots) and are used to facilitate maintainance,
cf. http://www.c0t0d0s0.org/archives/4372-New-features-of-Solaris-Alternate-boot-environments-based-on-snapshots.html.

If so, maybe you can salvage code from there (or Illumos)? 

Cheers,
-- 
Christopher J. Ruwe
TZ GMT + 1