Data distribution after adding a 2nd VDEV

[purplePeacock]

Hi,
I'm relatively new to TrueNAS stuff, I've been working with it for a few years with a low capacity pool consiting of:

3x4TB in one raidz1 vdev

I am looking to double my capacity by adding a second VDEV that is identical to my current vdev and I am wonder how the data is distributed after I add the vdev.
Specifically I have these questions:

1. Is data added after the expansion striped onto both vdevs or does the first existing vdev fill up first and then the second.
2. If the new data added after the pool expansion is striped across the two vdevs (likely improving read/write performance), what happens to the data that was added before the expansion that is on only the first vdev? Is it moved around so half of it is on the new vdev or does that data just stay on the original vdev and have 1/2 the read speed of data that is added after the new vdev is added?

I don't know if that will make sense or not so let me know if I need to clarify.

Thanks in advance.

 

[sretalla]

Is data added after the expansion striped onto both vdevs or does the first existing vdev fill up first and then the second.

Neither... data will remain where it is, but with both VDEVs available, data will continue to add to both (perhaps with some skewing toward the emptier VDEV, but no specific process of evening out).

Unless you either re-create your pool and add the data back to it or use a rebalancing script, you're stuck in the mess of unpredictable performance.

GitHub - markusressel/zfs-inplace-rebalancing: Simple bash script to rebalance pool data between all mirrors when adding vdevs to a pool.

Simple bash script to rebalance pool data between all mirrors when adding vdevs to a pool. - markusressel/zfs-inplace-rebalancing

github.com