Replacing UNAVAIL devices

[rlaager]

As discussed here:
http://groups.google.com/a/zfsonlinux.org/group/zfs-discuss/browse_thread/thread/c515f1d9e244b31a

It should be possible to replace a missing device without hacks such as creating fake device files.

[behlendorf]

Agreed. This sounds like probably a pretty easy thing to fix once someone digs in to it.

Instead of creating symlinks to /dev/null or such, one can simply use the guid of the missing drive from the output of 'zdb'.

[FransUrbo]

In rc9, I did not have to fiddle with links or empty files to replace a UNAVAIL device. I did, however, have to specify the full path (/dev/disk/by-id/XXXX) to both the old (not longer availible disk) and the new. So it partly works as expected.

[behlendorf]

For those people interested in this issue could you please test #1056. I'd appreciate the feedback, it resolves the issue in my testing but I'd like for the patch to get a little more real world abuse.