<h1>Root on ZFS: A ZPool of Mirror VDEVs</h1>
<p class="description">
I wanted/needed to make a root on ZFS pool out of multiple mirror VDEVs,
and since I'm not a ZFS expert, I took a little shortcut.
</p>
<p>
I recently got a new-to-me server (yay!) and I wanted to do a
root-on-ZFS setup on it. I've really enjoyed using ZFS for my data
storage pools for a long time. I've also enjoyed the extra functionality
that comes with having a bootable system installed on ZFS on my laptop
and decided with this upgrade it's time to do the same on my server.
Historically I've used RAIDZ for my storage pools. RAIDZ functions
almost like a RAID10 but at the ZFS level. It gives you parity so that a
certain number of disks can die from your pool and you won't lose any
data. It does have a few tradeoffs however*, and for personal
preferences I've decided that for the future I would like to have a
single ZPool over top of multiple mirror VDEVs. In other words, my main
root+storage pool will be made up of two-disk mirrors and can be
expanded to include any number of new mirrors I can fit into the
machine.
</p>
<p>
This did present some complications. First of all,
<code>bsdinstall</code> won't set this up for you automatically (and
sure enough,
<a href="https://www.freebsd.org/doc/handbook/bsdinstall-partitioning.html">in the handbook</a>
it mentions the guided root on ZFS tool will only create a single,
top-level VDEV unless it's a stripe). It will happily let you use RAIDZ
for your ZROOT but not the more custom approach I'm taking. I did
however use
<code>bsdinstall</code> as a shortcut so I wouldn't have to do all of
the partitioning and pool setup manually, and that's what I'm going to
document below. Because I'm totally going to forget how this works the
next time I have to do it.
</p>
<p>
In my scenario I have an eight-slot, hot-swappable PERC H310 controller
that's configured for AHCI passthrough. In other words, all FreeBSD sees
is as many disks as I have plugged into the backplane. I'm going to fill
it with 6x2TB hard disks which, as I said before, I want to act as three
mirrors (two disks each) in a single, bootable, growable ZPool. For
starters, I shoved the FreeBSD installer on a flash drive and booted
from it. I followed all of the regular steps (setting hostname, getting
online, etc.) until I got to the guided root on ZFS disk partitioning
setup.
</p>
<p>
Now here's where I'm going to take the first step on my shortcut. Since
there is no option to create the pool of arbitrary mirrors I'm just
going to create a pool from a single mirror VDEV of two disks. Later I
will expand the pool to include the other two mirrors I had intended
for. My selections were as follows:
</p>
<ul>
<li>Pool Type/Disks: mirror mfisyspd0 mfisyspd1</li>
<li>Pool Name: zroot</li>
<li>Partition Scheme: GPT (EFI)</li>
<li>Swap Size: 4g</li>
</ul>
<p>
Everything else was left as a default. Then I followed the installer to
completion. At the end, when it asked if I wanted to drop into a shell
to do more to the installation, I did.
</p>
<p>
The installer created the following disk layout for the two disks that I
selected.
</p>
<pre>
<code>
atc@macon:~ % gpart show
=> 40 3907029088 mfisyspd0 GPT (1.8T)
40 409600 1 efi (200M)
409640 2008 - free - (1.0M)
411648 8388608 2 freebsd-swap (4.0G)
8800256 3898228736 3 freebsd-zfs (1.8T)
3907028992 136 - free - (68K)
=> 40 3907029088 mfisyspd1 GPT (1.8T)
40 409600 1 efi (200M)
409640 2008 - free - (1.0M)
411648 8388608 2 freebsd-swap (4.0G)
8800256 3898228736 3 freebsd-zfs (1.8T)
3907028992 136 - free - (68K)
</code>
</pre>
<p>
The installer also created the following ZPool from my single mirror
VDEV.
</p>
<pre>
<code>
atc@macon:~ % zpool status
pool: zroot
state: ONLINE
scan: none requested
config:
NAME STATE READ WRITE CKSUM
zroot ONLINE 0 0 0
mirror-0 ONLINE 0 0 0
mfisyspd0p3 ONLINE 0 0 0
mfisyspd1p3 ONLINE 0 0 0
errors: No known data errors
</code>
</pre>
<p>
There are a couple of things to take note of here. First of all,
<em>both</em> disks in the bootable ZPool have an EFI boot partition.
That means they're both a part of (or capable of?) booting the pool.
Second, they both have some swap space. Finally, they both have a third
partition which is dedicated to ZFS data, and that partition is what got
added to my VDEV.
</p>
<p>
So where do I go from here? I was tempted to just
<code>zpool add mirror ... ...</code> and just add my other disks to the
pool (actually, I <em>did</em> do this but it rendered the volume
unbootable for a very important reason), but then I wouldn't have those
all-important boot partitions (using whole-disk mirror VDEVS). Instead,
I need to manually go back and re-partition four disks exactly like the
first two. Or, since all I want is two more of what's already been done,
I can just clone the partitions using <code>gpart backup</code> and
<code>restore</code>! Easy! Here's what I did for all four remaining
disks:
</p>
<pre>
<code>
root@macon:~ # gpart backup mfisyspd0 | gpart restore -F mfisyspd2`
</code>
</pre>
<p>
Full disclosure, I didn't even think of this as a possibility
<a href="ihttps://unix.stackexchange.com/questions/472147/replacing-disk-when-using-freebsd-zfs-zroot-zfs-on-partition#472175">until I read this Stack Exchange post</a>. This gave me a disk layout like this:
</p>
<pre>
<code>
atc@macon:~ % gpart show
=> 40 3907029088 mfisyspd0 GPT (1.8T)
40 409600 1 efi (200M)
409640 2008 - free - (1.0M)
411648 8388608 2 freebsd-swap (4.0G)
8800256 3898228736 3 freebsd-zfs (1.8T)
3907028992 136 - free - (68K)
=> 40 3907029088 mfisyspd1 GPT (1.8T)
40 409600 1 efi (200M)
409640 2008 - free - (1.0M)
411648 8388608 2 freebsd-swap (4.0G)
8800256 3898228736 3 freebsd-zfs (1.8T)
3907028992 136 - free - (68K)
=> 40 3907029088 mfisyspd2 GPT (1.8T)
40 409600 1 efi (200M)
409640 2008 - free - (1.0M)
411648 8388608 2 freebsd-swap (4.0G)
8800256 3898228736 3 freebsd-zfs (1.8T)
3907028992 136 - free - (68K)
=> 40 3907029088 mfisyspd3 GPT (1.8T)
40 409600 1 efi (200M)
409640 2008 - free - (1.0M)
411648 8388608 2 freebsd-swap (4.0G)
8800256 3898228736 3 freebsd-zfs (1.8T)
3907028992 136 - free - (68K)
=> 40 3907029088 mfisyspd4 GPT (1.8T)
40 409600 1 efi (200M)
409640 2008 - free - (1.0M)
411648 8388608 2 freebsd-swap (4.0G)
8800256 3898228736 3 freebsd-zfs (1.8T)
3907028992 136 - free - (68K)
=> 40 3907029088 mfisyspd5 GPT (1.8T)
40 409600 1 efi (200M)
409640 2008 - free - (1.0M)
411648 8388608 2 freebsd-swap (4.0G)
8800256 3898228736 3 freebsd-zfs (1.8T)
3907028992 136 - free - (68K)
</code>
</pre>
<p>
And to be fair, this makes a lot of logical sense. You don't want a
six-disk pool to only be bootable by two of the disks or you're
defeating some of the purposes of redundancy. So now I can extend my
ZPool to include those last four disks.
</p>
<p>
This next step may or may not be a requirement. I wanted to overwrite
where I assumed any old ZFS/ZPool metadata might be on my four new
disks. This could just be for nothing and I admit that, but I've run
into trouble in the past where a ZPool wasn't properly
exported/destroyed before the drives were removed for another purpose
and when you use those drives in future
<code>zpool import</code>s, you can see both the new and the old, failed
pools. And, in the previous step I cloned an old ZFS partition many
times! So I did a small <code>dd</code> on the remaining disks to help
me sleep at night:
</p>
<pre>
<code>
root@macon:~ # dd if=/dev/zero of=/dev/mfisyspd2 bs=1M count=100
</code>
</pre>
<p>
One final, precautionary step is to write the EFI boot loader to the new
disks. In
<a href="https://www.freebsd.org/doc/handbook/zfs-zpool.html">zpool admin handbook</a>
it mentions you should do this any time you <em>replace</em> a zroot
device, so I'll do it just for safe measure on all four additional
disks:
</p>
<pre>
<code>
root@macon:~ # gpart bootcode -p /boot/boot1.efifat -i 1 mfisyspd2
</code>
</pre>
<p>
Don't forget that the command is different for UEFI and a traditional
BIOS. And finally, I can add my new VDEVs:
</p>
<pre>
<code>
root@macon:~ # zpool zroot add mirror mfisyspd2p3 mfisyspd3p3
root@macon:~ # zpool zroot add mirror mfisyspd4p3 mfisyspd5p3
</code>
</pre>
<p>And now my pool looks like this:</p>
<pre>
<code>
atc@macon:~ % zpool status
pool: zroot
state: ONLINE
scan: none requested
config:
NAME STATE READ WRITE CKSUM
zroot ONLINE 0 0 0
mirror-0 ONLINE 0 0 0
mfisyspd0p3 ONLINE 0 0 0
mfisyspd1p3 ONLINE 0 0 0
mirror-1 ONLINE 0 0 0
mfisyspd2p3 ONLINE 0 0 0
mfisyspd3p3 ONLINE 0 0 0
mirror-2 ONLINE 0 0 0
mfisyspd4p3 ONLINE 0 0 0
mfisyspd5p3 ONLINE 0 0 0
errors: No known data errors
</code>
</pre>
<p>
Boom. A growable, bootable zroot ZPool. Is it easier than just
configuring the partitions and root on ZFS by hand? Probably not for a
BSD veteran. But since I'm a BSD layman, this is something I can live
with pretty easily. At least until this becomes an option in
<code>bsdintall</code> maybe? At least now I can add as many more
mirrors as I can fit into my system. And it's just as easy to replace
them. This is better for me than my previous RAIDZ, where I would have
to destroy and re-create the pool in order to add more disks to the
VDEV. Now I just create another little mirror and grow the pool and all
of my filesystems just see more storage. And of course, having ZFS for
all of my data makes it super easy to create filesystems on the fly,
compress or quota them, and take snapshots (including the live ZROOT!)
and send those snapshots over the network. Pretty awesome.
</p>
<p>
* I'm not going to explain why here, but
<a href="http://www.openoid.net/zfs-you-should-use-mirror-vdevs-not-raidz/">this is a pretty well thought out article</a>
that should give you an idea about the pros and cons of RAIDZ versus
mirror VDEVs so you can draw your own conclusions.
</p>
