Mirrored pair of different size VDEVs

[Kean]

My FreeNAS setup:
MB: SuperMicro X10SDV-2C-7TP4F
CPU: embeded Pentium/Xeon D-1508
RAM: 2x16GB ECC DDR4
Boot pool: 2x16GB SuperMicro SATADOM
HDD: 6 x 3TB WD RED + 2 x 480 GB Intel S3610
Pool setup: RAID Z2 for HDD and Mirror for SSD

I have mirrored SSD drives as a pool for iSCSI VM storage. This storage contain around 8 VM images and frankly speaking it's not super loaded pool. But, i'm running out of space. I'm passed 50% capacity and there in some rare cases VM storage migration is complaining about low disk space for temp files.
I decided to add another pair of SSD as a new VDEV. I would like to get Intel S3610 800GB.

Now 2 questions:
1. Do you still recommend to get enterprise SSDs rather than consumer grade?
2. Main question: How does ZFS handles such pool? It will have 480+800GB size. My view on this is that files will be resilvered and spread across both pairs. Or Will they be stored in some proportion of the sizes of the VDEVs?
And then, when there is more files how ZFS will handle split? Will it populate the space in same proportion as size of the VDEVs or split 1:1 by end of capacity for smaller VDEV and then will bigger VDEV on top?

 

[sretalla]

1. Do you still recommend to get enterprise SSDs rather than consumer grade?

For enterprise reliability and longevity, yes.

2. Main question: How does ZFS handles such pool?
It will have 480+800GB size. My view on this is that files will be resilvered and spread across both pairs. Or Will they be stored in some proportion of the sizes of the VDEVs?

No realignment of the data will happen automatically.

ZFS will decide which VDEV files will be placed on internally (generally will favour the new/mostly empty one).

You can cause a realignment by emptying the pool and copying the contents back to it. (see previous sentence for how that will work)

And then, when there is more files how ZFS will handle split?

As said, ZFS will decide, but as a general rule it will try to balance, so in your case perhaps 2/3 of the files will go to the larger drives... you should not care about this as you can't use that information for anything and those drives/that VDEV can't be used without the whole pool, so it should not matter.