FreeBSD: ZFS pool using GPT labels

From this point on, I will never again create a ZFS pool without using GPT labels. Why? Because it makes life easier down the road - especially during operations such as adding or replacing disks.

While this post focuses on FreeBSD, the same technique should work on any Unix system using GPT and ZFS.

Overview

The “common” way of creating a ZFS pool involves something along the lines of:

zpool create tank mirror ada0 ada1

This will create a new zpool named tank using devices /dev/ada0 and /dev/ada1 in a mirrored configuration. So far so good.

There are mainly two problems apparent with this:

  • The disk identifiers (eg. ada0) can change at any time - especially when adding, removing or changing disks on the host.
  • Identifying the physical disk is… tricky.

GPT (GUID Partition Table) allows to label individual partitions. Once a label has been assigned to a partition, it will show up as /dev/gpt/<label>. Therefore, a particular partition can be accessed by label instead of by device, device driver or other means.

Using a GPT label allows to physically label the disk with the same label. Therefore, identifying which disk needs replacement is very easy. Without this 1-to-1 labelling, identifying the physical disk can be difficult as one would need to know to which physical port the disk is connected.

Creating a ZFS pool using GPT labels

Creating a ZFS pool which uses GPT labels is a simple process. However, it requires manually creating the GPT, the ZFS partition and the corresponding label for every device that will be part of the ZFS pool.

From hereon we assume that a mirror pool is created using two 512GB SATA SSDs which show up as /dev/ada0 and /dev/ada1.

For every device to be added to the pool, we first create a GPT:

gpart create -s GPT /dev/ada0

Next, we create the ZFS partition. At this point it is worth considering making the partition slightly smaller than the actual capacity available on the disk. In my specific case the SSDs report 477GB of usable space. Imagine having to replace a 512GB SATA SSD with a 512GB SATA SSD from a different manufacturer. It might happen that the replacement disk has a little bit less usable space (eg. 475GB instead of 477GB). In that case, you’re gonna find yourself in a world of pain quickly. Leaving a bit of empty space mitigates issues like this easily.

I’ll create partitions that are 470GB in size using 4k sectors:

gpart add -t freebsd-zfs -a 4k -s 470g ada0

Then, add a label to the newly created ZFS partition:

gpart modify -l disk0 -i 1 ada0

Repeat this procedure for the remaining disk(s) using a different label for each.

After each disk has been prepared, the ZFS pool can be created. Here, we create a two-way mirror named ssd.

zpool create ssd mirror /dev/gpt/disk0 /dev/gpt/disk1 

If everything succeeded and the pool has been created, zpool status should show the pool with the corresponding GPT labels:

  pool: zroot
 state: ONLINE
config:

        NAME           STATE     READ WRITE CKSUM
        ssd            ONLINE       0     0     0
          mirror-0     ONLINE       0     0     0
            gpt/disk0  ONLINE       0     0     0
            gpt/disk1  ONLINE       0     0     0

errors: No known data errors

And there you have it - a ZFS pool using GPT labels! The resulting ZFS pool (more?) is resilient towards changing disk’s physical locations and adding/removing other disks on the same host.

Replacing disks is as easy as using zpool status to figure out which disk needs replacement and looking for the same label on the physical disk (or drive bay).

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