[tjoff]

Does the job though.

Nope.

It will result in very uneven utilization which leads to poor performance. Not recommended. Is also extremely wasteful. The whole point of the sought feature is to minimize waste and cost. So no, I'd strongly disagree that it does the job.

[gruturo]

I'm not trying to defend ZFS' lack of the ability to add a vdev to an existing RAIDZ/Z2. That would be extremely nice to have and has tons of legitimate uses, for many of which there is no acceptable substitute as you point out.

That said, even if that function existed, expecting it to magically rebalance the whole pool to take into account the existing data is rather unrealistic (it's basically a full rebuild in that case, the most terrifying operation you can perform on any pool/array already containing data).

While I agree with the performance aspects (your data still comes from either the first volume or the second, so you're only using the performance of half your spindles - unless you simultaneously access files in either half), I wouldn't call it wasteful: You are still losing the same % of your total capacity. e.g.:

6x2TB RaidZ2 = 8TB usable (4TB lost to parity)

let's add a second Z2 6x6TB volume:

6x6TB RaidZ2 = 24TB usable (12 TB lost to parity)

Total: 32 TB usable, 16 lost to parity.

What if we had built it from scratch using 6x8TB drives?

Total: 32 TB usable, 16 lost to parity.

Same.

[tjoff]

That said, even if that function existed, expecting it to magically rebalance the whole pool to take into account the existing data is rather unrealistic

That is exactly what is required (well, you re-balance the vdev, not the pool) and is what is proposed and has been in the works for basically forever. SUN never prioritized this because it has few uses within the enterprise (but extremely valuable for home users).

Your example is quite misleading. Here's is a much more common and realistic scenario.

6x2TB RaidZ2 = 8 usable, 4 lost to parity.

Now, imagine that you want to expand the pool with 4 TB. Being able to add to a raidz vdev would result in this:

8x2TB RaidZ2 = 12 Usable, 4 lost to parity. (ridiculously cheap, no further waste at all)

Current scenario:

6x2TB RaidZ2 + 4x2TB RaidZ2 = 12 Usable, 8 lost to parity. (expensive, lower performance, more power, more noise, requires more harddrive slots (this is often a very important aspect for home-setups))

The waste is outrageous. And the consequences from this waste reflects every aspect of designing a home-setup (as to avoid the above scenario) with ZFS and is vastly different to how you would design a system with, for instance, raid6 with expansion in mind.

[tscs37]

The comparison is a bit off.

If you build a RAIDZ2 wiht a 3x8TB and a 3x8TB pool, you loose 2+2x8TB = 32TB and have only 16TB usuable.

6x2+6x6 is a mixed pool so it's hard to compare to actual disks.

An alternative, better, comparison, would be a 6x8TB pool and another 6x8TB pool.

This time you loose 32TB to parity and have 8x8=48TB usuable. If you rebuild it to a 12x8TB pool, you loose 16TB to parity and have 80TB usuable.