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author53hornet <atc@53hor.net>2021-07-28 10:58:58 -0400
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- <article>
- <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>
- </article>
- </body>
-</html>