Curious question about using zfs send -R and receive on FreeBSD

[Christopher J. Ruwe]

On Mon, 17 Dec 2012 16:00:06 -0600
dweimer <dweimer at dweimer.net> wrote:

> I recently migrated a machine that was built on a VM to physical 
> hardware using the zfs send -R option against a snapshot of its root
> zfs setup.  I went from smaller drives to larger, both using a
> mirrored zpool.  However the devices were different, the device IDs
> on the VM were da0 and da1, the device IDs on the physical hardware
> were ada0 and ada1.  I had used labels when creating the gpt layout
> to plan for this. And all worked great, in fact it was the fastest I
> have ever converted a virtual machine to a physical machine.
> After I finished  though, I got curious, was it actually necessary
> for me to mount the new boot zfs partition while running on the live
> cd and copy the zpool.cache file I had created when creating the
> zpool or would have the existing cache file that would have been
> included in the zfs send contained the right information?  As the
> zpool was pointed at the gpt label devices, or was the fact that the
> size changed enough difference that copying the file was indeed
> necessary?
> 

I fear you might be mixing things up here. You did a zfs-migrate, not a
zpool-migrate, you created a new zpool and received the datasets on that
new zpool, even if the zpool names were the same.

The function of zpool.cache is to tell the OS about available
device-zpool combinations. Without, the OS does not know which zpools
are available and accordingly cannot mount the root-ZFS passed in the
vfs.root.mountfrom directive in loader.conf, which is why you needed to
copy the zpool.cache and, if you changed the zpool-names, needed to
adapt loader.conf.

There are plans to change this behaviour, as it is deemed superfluous at
least in the case of disks, but I do not know how much that has
progressed so far. 

http://lists.freebsd.org/pipermail/freebsd-fs/2012-October/015328.html


Hope I could shed some light on that issue, although I am by no means an
expert on this.

Cheers,
-- 
Christopher
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