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From: Artem Belevich <art@freebsd.org>
To: Peter Jeremy <peterjeremy@acm.org>
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Subject: Re: "can't load 'kernel'" on ZFS root
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On Tue, Sep 6, 2011 at 9:48 PM, Peter Jeremy <peterjeremy@acm.org> wrote:
> Secondly, I found that ZFS booting is extremely fragile - if your
> default kernel & modules won't load automatically, there's no way to
> boot the backup kernel because (as far as I can find) there's no way
> to manually load /boot/zfs/zpool.cache - without which the kernel
> can't mount the root FS. =A0In both above cases, recovering the system
> required booting from recovery media. =A0There needs to be a documented
> method for booting from a snapshot or clone.

The magic command is:

load -t /boot/zfs/zpool.cache  /boot/zfs/zpool.cache

It makes me wonder, though -- if we're probing devices anyways, why is
zpool.cache existence mandatory? According to the name it's a *cache*,
presumably to speed up zpool detection on a normal boot. Perhaps we
can fall back to probing all drives if zpool.cache is missing. Slower
boot definitely beats no booting at all.

--Artem