Manual setup Z2 or default 2 mirror vdevs?

[krizzo]

I'm creating a new pool as my current pool raidz2 8x2T drives is nearing 90%.

I have four 6T drives and two 5T drives. I keep debating which setup I want to go with as I do plan on replacing the two 5T drives with 6T drives in the future.
I figured by the time this new pool gets close to full I'll have upgraded the 5Ts and my 2T pool might be swapped with who knows what size.

What option would those of you who are more experienced with zfs and pools along with migration and upgrading suggest?

1. Raidz2 all six drives, losing 4T of storage until the 5Ts are upgraded, this starts out at 17T and will grow to 20T. I've read that you get a performance hit mixing different devs in a vdev.
   
2. Two vdev mirrors 2x2x6T and 2x1x5T, This one bothers me due to my plan on upgrading it would restrict the total capacity to 15T then only grow to 16T.

 

[Linkman]

Option 1. RAIDZ2 the six drives, and replace the 5TB as planned.

You begin with more space than you would have after growing option 2, and you don't seem to need the IOPS of the mirrored vdevs, since you are coming from RAIDZ2 already.

 

[garm]

Agreed, the vdev layout is primary to other considerations in this case in my opinion

 

[SweetAndLow]

Raidz2