Right now i have a Pool 1 x RAIDZ1 | 4 wide | 14.55 TiB
I bought 2 new 16TB (14.55TiB) and i want to expand it (5HDD for storage and 1 for Parity).
I see the Extend Vdev option where i can add my 2 new HDDs right? but also says this
The expanded vdev uses the pre-expanded parity ratio, which reduces the total vdev capacity. To reset the vdev parity ratio and fully use the new capacity, manually rewrite all data in the vdev. This process takes time and is irreversible.
What exactly does that mean, right now i have 21TB occupied with data
Maybe it’s easier to understand with an example:
Let’s say you stored 10TB in your 4-wide RAIDZ1. Due to parity, you actually need ~13.3TB of disk space (Very rough approximations here).
However if you store 10TB in a 6-wide RADIZ1, you only need ~12TB of disk space, saving you 1.3TB of disk space when compared to the 4-wide RAIDZ1.
Due to the way ZFS expansion works you don’t actually get any of that disk space back immediately. You need to rewrite the data if you want to make use of the space savings of the new parity ratio.
Please note that these “space savings” of course aren’t free. Wider pools are more suspectible to failures due to having higher amount of drives.
