Choosing drives for mirror

[Jurisd]

Hello - I'm trying to set up my first FreeNAS server, and am at the stage of creating the volume. I have two 4TB WD Red SATA drives that are brand new. I have two old 4TB external drives (each with roughly 15,000 hours on them). My initial use case is as a home media server, where all of the content will be elsewhere (on other computers). So although redundancy/backup is not a critical consideration, I would like to set this up so that I have a reliable backup (thinking mirror) so that I don't have to go "recollect" everything from various places to reconstruct the media vault in the event of failure. I also may later use some of the NAS space for family pictures where backup would be more critical, so I want to set this up in case I do that later. I am thinking the best configuration is to set up a 2 drive main, and a 2 drive mirror. BUT, here's the issue (probably reflects how little I understand so far about ZFS and RAID...)
I think it makes the most sense for the two internal, new, reliable drives to be the main storage pool, and the two external, old USB drives to be the mirror. Am I correct on that? If so, how do I configure the volume to do that? Currently when I'm trying to create the volume, it shows a stack with da0 and da1 (the external drives) on the top row of a 2x2 matrix, and ada1 and ada2 (the two internal HDs) on the bottom row. Does the top row vs bottom row indicate which are the "primary" storage? Or, am I trying to do FreeNAS' job for it, and should I just let it decide which disks do what (and then in that case, should I choose RAID Z2?)

Thanks, and sorry for the long-winded question...

 

[danb35]

You need to get rid of the concepts of primary storage vs. the mirror, as they don't apply here. With a mirrored pool, in the simplest case, you have two disks with the same data on them. Whatever is written to one is simultaneously written to the other. Barring a catastrophic failure, there should never be a time when the contents of the two disks are not identical.

With four disks, as you're considering (and in the sort of configuration you're considering), you have two mirrored pairs of disks, each of which works as I've just described. Data is then striped across those two pairs of disks.

Do not, in any event, use external USB drives for any kind of long-term use.

 

[garm]

Also, in no sense of the word is this backup. It’s redundancy as there are single event catastrophes that can take out all your data.