[zfs-discuss] Boot Mirror

Chris Siebenmann cks at cs.toronto.edu
Tue Apr 12 21:30:01 EDT 2016

> [...] I guess the easiest ways to get tripped up are managing the
> bootloader. By just attaching a mirror to rpool, OI automatically
> updates grub for you on the target disk. If one disk dies, you can
> boot from either disk, and if you reattach the other disk later, zfs
> resilver automatically knows which one is later, and brings them into
> sync.
> On linux, do you have to manually install grub on the replacement
> disk?  I'm guessing yes.

 I believe so.

> According to some sources on the internet
> (https://help.ubuntu.com/community/Installation/SoftwareRAID) it looks
> like you can't boot from a degraded disk. You have to boot into a
> livecd, edit the mdadm file to permit booting from degraded.

 You can override that on install or later and any server install
should (it's so much a part of our install system that I forget it's there
as a stock default). The Ubuntu default is crazy for server operation
and I don't think other distributions follow it.

(I believe you may even get asked this during Ubuntu installs if you
set up a RAID-1 mirror, and who knows, Ubuntu may even have changed the
default in 16.04 for server installs.  It would be about time.)

 I don't know how distributions handle updating boot blocks when grub
is updated. However, as far as I know most of a modern grub setup is
actually in the filesystem under /boot, so even the need for bootblock
updates is probably fairly rare.

(Btw, I wouldn't bother with a separate /boot these days. Just roll it
into the root filesystem; it works just as well and makes life simpler.)

	- cks

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