SOLVED Can I take disks from half empty pool to expand 2nd pool (all mirrored vdevs)?

[sammael]

Hi,

I’m running TrueNAS-12.0-U4. I have 2 pools of mirrored vdevs (8x6TB and 8x4TB disks). Due to my absolute failure to plan anything properly pool1 is at 78% and pool2 is barely half full.

Both pools started as 2 or 4 disks and as good hdd prices popped up I kept adding another pair of vdevs. I can’t currently afford new pair of disks for pool1, so I was wondering if it’s safe to do this:

Can I just disconnect 2 disks from pool2 in the Truenas UI and then import them as 5th mirrored vdev to pool1? It’s my layman understanding that by using mirrored vdevs and not zraid, I should get additional ~4TB of space on pool1 even if the other 4 pair of vdevs are 6TB, yes?

Thanks for any advice!

 

[danb35]

Can I just disconnect 2 disks from pool2 in the Truenas UI and then import them as 5th mirrored vdev to pool1?

Yes. Go to the pool status page of pool2, click the kebab menu next to the pair of disks you want to remove, and select Remove. Looks like this:

I should get additional ~4TB of space on pool1 even if the other 4 pair of vdevs are 6TB, yes?

No, if you add a mirrored pair of 6 TB disks, you'd add about 6 TB capacity to your pool.

 

[sammael]

Thanks for the confirmation!

To clarify:
pool1 is 4x pair of 6TB
pool2 is 4x pair of 4TB

So if I take 1 pair of 4TB disks and add them to the 4 pairs of 6TB disks, it should not cause any issue because of the size mismatch and I should get additional 4TB on pool1?

 

[QonoS]

Within a pool you can have mixed sized VDEVs. So you can have:

• mirror-0 (4T+4TB) => 4TB
• mirror-1 (6TB+6TB) => 6TB
• mirror-2 (2TB+2TB) => 2TB
• raidz2-0 (1TB+1TB+1TB+1TB+1TB+1TB) => 4TB
• etc...

But disks being part of a single VDEV should all have the exact same size.

So 4TB+4TB is perfectly fine addition for a mirrored VDEV.

 

[danb35]

But disks being part of a single VDEV should all have the exact same size.

...and even this isn't mandatory, but "extra" space on a larger disk will be wasted.

 

[sammael]

Great stuff that clears it up, thanks! Feel a lot safer tinkering with it now.

 

[Arwen]

Backup!

When ever changing pool layout, if you can perform a full & complete backup, do so.

We have seen people who remove the wrong disk(s), and then have an additional disk fail on power up after the other disk removal. So, let's return to my first word of this post, Backup!