Hi there,

I currently have a 2TB mirrored ZFS pool (comprising two SATA SSDs) that I use as my backup target. It's now filling up, and as I don't have any more NVMe ports, nor PCI-e slots, I have just purchased 2x more 2TB SATA SSDs.

My initial intention was to move to RAIDZ1, but I heard that there's a performance hit, and I found some (now discontinued) MX500 SSDs for a great price, so I thought I would add another Mirrored vdev to the pool.

Am I correct in thinking it's possible to do this? Maintain a single pool in Unraid, but with two sets of mirrored disks?

The documentation here seems to say that it's possible, but I'm unsure if I have interpreted it correctly, or how I go about installing the new disks so that the storage of the pool appears seamless between the two mirrors.

To quote:

Ways to expand your pool include:

• Adding a new vdev: Grow your pool by adding another vdev (like a new mirror or RAIDZ group). This increases capacity, but you must add disks in sets that match the vdev’s configuration.

So how do I go about achieving this?

Do I stop the array, add 2x more slots to my "pool2tb" then just add the two new disks? Or is another step required?

Your help is appreciated.

Below are the "Pool status"

pool: pool2tb
 state: ONLINE
  scan: scrub repaired 0B in 01:15:09 with 0 errors on Mon Sep  8 01:15:10 2025
config:

	NAME                  STATE     READ WRITE CKSUM
	pool2tb               ONLINE       0     0     0
	  mirror-0            ONLINE       0     0     0
	    /dev/mapper/sdc1  ONLINE       0     0     0
	    /dev/mapper/sdb1  ONLINE       0     0     0

errors: No known data errors

And "Pool Information"

NAME         SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP    HEALTH  ALTROOT
pool2tb     1.81T  1.41T   413G        -         -     4%    77%  1.00x    ONLINE  -
  mirror-0  1.81T  1.41T   413G        -         -     4%  77.8%      -    ONLINE
    sdc1    1.82T      -      -        -         -      -      -      -    ONLINE
    sdb1    1.82T      -      -        -         -      -      -      -    ONLINE

Many thanks.

1 hour ago, jademonkee said:

Am I correct in thinking it's possible to do this? Maintain a single pool in Unraid, but with two sets of mirrored disks?

Yep, just add two slots to the pool, assign both devices and start the pool.

19 hours ago, JorgeB said:

Yep, just add two slots to the pool, assign both devices and start the pool.

Just reporting back to say that it worked like a charm: everything handled in the back end, and I now have a 4TB (4x 2TB) mirrored pool. No extra config or fiddling was needed beyond just assigning the two new disks to the two new slots in the existing pool.

Many thanks for your help - and a big shout out to Unraid and the Unraid devs: I'm continually impressed by the simplicity of managing this OS.