Different Vdevs in same pool

[TheDubiousDubber]

So I read somewhere that there can be performance drawbacks to having multiple vdevs that are not of the same size/type or whatever. I currently have a 6x2TB RAIDZ2. I also have a separate backup server I've been putting together with a 6x4TB pool in anticipation of expanding my main pool when I get around to it. My question is, what are the drawbacks, if any, to a 4x4TB RAIDZ2 and adding to my main pool rather than more smaller drives like another 6x2TB?

 

[Arwen]

Before we get started, some people think that non-symetrical VDEVs is both bad and something to fix.
Especially if you plan on high performance, (10Gbps), or need low latency, (like for VMware).

My own opinion is that they need to use the same level of redundancy, (Mirror, RAID-Z2, etc...). Plus,
you need to understand the performance may be irregular. That should not be a problem for a home
or small office NAS.

Now, from what I know, the situation is that if your first VDEV, (6 x 2TB in RAID-Z2), is fullish, then
most new data, (including copy-on-writes), will end up on the second VDEV, (4 x 4TB in RAID-Z2).
So any reads of data from the first VDEV could be stripped across 4 disks. But any reads from the
second VDEV would be from just 2 disks.

All that said, it's a performance issue that if you can live with it, my opinion is that it will work fine.
Modern NAS disks are reasonably fast, and many people are limited to 1Gbps speeds. If you were
trying the above mentioned VMware or needed high performance Ethernet, or low latency, then
you want symetrical. And even preferably the same size.