Add a Mirror to an Existing ZFS Drive

One of my criticisms of ZFS is that it’s not flexible: Once you set up a dataset, you’re pretty much stuck with it. But this isn’t entirely true: You can add a mirror or stripe to an existing pool without destroying the data. Since this is quite a common operation, I decided to write it up for posterity.

A Very Simple ZFS Pool

Let’s say you built the world’s most basic ZFS pool: A single disk. Then you learned that this is a Very Bad Idea and want to add some data protection. It’s actually quite simple to do, though it does require the command line in FreeNAS.

Here’s our setup:

We have a single disk attached to our system, /dev/da1
This disk is used as a simple zfs pool, tank
You bought another disk and want to mirror the pool to the new disk, /dev/da2

First, make sure you’ve got the disk names correct. I use dmesg to see the system messages, which usually make it pretty obvious which disk is which. This process is only non-destructive if you specify the correct disk drives. If you’re not sure, do not continue!

Prepare Your Drive

My drive was formatted from the factory, so I had to delete the original partitions:

sudo gpart destroy -F /dev/da2

Then I formatted the drive as gpt:

sudo gpart create -s gpt /dev/da2

And created a zfs partition:

sudo gpart add -t freebsd-zfs /dev/da2

This new partition is /dev/da2p1, and I’ll add it to my existing partition in a moment.

Get the Partition IDs

Next we need to get the information we’ll feed zfs so it will configure the drives correctly.

We will need the gpt ID of the existing partition as well as the new one we’re adding. The “zpool status” command will show all we need to know about the existing drive, partition, and pool.

$ zpool status

  pool: Internal
 state: ONLINE
  scan: none requested
config:

        NAME                                          STATE     READ WRITE CKSUM
        Tank                                          ONLINE       0     0     0
          gptid/a8675309-b52e-12e6-a3fa-c13ed46d88b1  ONLINE       0     0     0

errors: No known data errors

See that bit after “gptid/”? That’s the unique ID of our existing zfs disk: “a8675309-b52e-12e6-a3fa-c13ed46d88b1”

Next we need to know the ID of the new partition we just created. Here’s an abbreviated output of “gpart list”:

$ gpart list

Geom name: da2
modified: false
state: OK
fwheads: 255
fwsectors: 63
last: 7814037127
first: 40
entries: 128
scheme: GPT
Providers:
1. Name: da2p1
   Mediasize: 4000786984960 (3.6T)
   Sectorsize: 512
   Stripesize: 4096
   Stripeoffset: 0
   Mode: r0w0e0
   rawuuid: 7d42ec0f-dfb7-12f7-a1fa-c13ee57d68b2
   rawtype: 517e7bbb-7ece-21d5-7fe8-00122d08722b
   label: (null)
   length: 4000786984960
   offset: 20480
   type: freebsd-zfs
   index: 1
   end: 7814037119
   start: 40
Consumers:
1. Name: da2
   Mediasize: 4000787029504 (3.6T)
   Sectorsize: 512
   Stripesize: 4096
   Stripeoffset: 0
   Mode: r0w0e0

Note that I deleted a lot of lines from this output!

The thing you need is the “rawuuid”. In my example, this is “7d42ec0f-dfb7-12f7-a1fa-c13ee57d68b2”

Putting It Together

Now we just need to tell zfs that these two disks need to be joined together as a mirrored set. Again, here are the inputs:

The pool name: Tank
The existing partition uuid: a8675309-b52e-12e6-a3fa-c13ed46d88b1
The new partition uuid: 7d42ec0f-dfb7-12f7-a1fa-c13ee57d68b2

So here’s the magic command:

sudo zpool attach Tank /dev/gptid/a8675309-b52e-12e6-a3fa-c13ed46d88b1 /dev/gptid/7d42ec0f-dfb7-12f7-a1fa-c13ee57d68b2

Now zfs will attach the new partition to the existing drive set, mirroring all data from the old drive to the new one. This process of copying data (“resilvering” in zfs parlance) will take a while. You can use the “zpool status Tank” command to see the progress, including an estimated time of completion. In my case (with two 4 TB drives containing a little over 1 TB of data) it took about 3 hours.

Note that zpool attach can only add mirrors to an existing zpool device. You can’t mess with RAID-Z sets in this manner, even though that would be much more useful. So don’t get your hopes up, but protect your data anyway!

© sfoskett for Stephen Foskett, Pack Rat, 2017. | Add a Mirror to an Existing ZFS Drive
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