I ask youcreating a pool for help.

I created a pool of SSD hard disks, but the total GB doesn’t add up, it should be 7.28 GB, instead the pool only gives me 3.51 GB, why?
I ask you for help.

It looks like you created a mirror vdev. In a mirror vdev each drive has an identical copy of the data, which protects you in case of a single drive failure.

A stripe vdev would be closer to what you are expecting in capacity, but is highly suggested against, if either drive fails all data is lost.

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how can I do it, it’s the first time I’ve done it.

I need the data to be saved on one disk, and the other acts as a backup in case of failure.

That looks to be what you have done. If you click on the gear in your first image and select Status it will give a better view of the pools layout.

Example of a mirror:

I did it like this

Yep, you have a mirror. Each disk has a copy of all data, in the event that one fails the pool will enter a degraded state and continue to function, but replace the failed disk as quickly as possible.

If you are new to ZFS, I highly suggest reading at the bare minimum our ZFS Primer.

ZFS has a lot of differences if you are coming from something like a Windows environment, and it is incredibly powerful but requires some research and understanding to fully get the benefits without exposing your data to undue risks.

OK perfect.
last thing, why is the total of the 2 disks not 7.28tb, but only 3.51tb?
It seems as if it didn’t add up the 2 disks, each disk has 4TB of capacity.
Why ?

Your drives are mirroring each other. They each have a copy of all data, therefore you cannot store more data than a single drive’s capacity.

so, is it correct, as I did, both for the mirror of the 2 disks, and for the capacity that truenas detects?


OK, perfect,
thank you very much for your help.
you’re fantastic, see you next time.

This is how you have it setup.

But this should not be your only backup. If the pool fails (for whatever reason) then you would still need a separate copy to prevent data loss.