June 2021 FreeBSD Developer Summit: Raid-Z Expansion

For those that like videos more than text, there is a youtube video from last year 1 that explain the feature (unless it's changed since ,but it seems not to be the case). One downside that I see of this approach, if I understand it correctly, is that the data already present on disk will not take advantage of the extra disk per slice. For example, if I have a raidz of 4 disks (so 25% of space "wasted"), and add another disk, new data will be distributed on 5 disks (so 20% of space "wasted") but the old data will keep using stripes of 4 blocks, they will just be reshuffled between the disks. Do I understand it correctly ? 1 link to this video

…sk and wrap around. So if your 5x10 TB disks are 90% full, after the expansion they will contain the same 5.4 TB of data and 3.6 TB of parity, and the pool will now be 10 TB bigger. New writes, will be 4+2 instead, but the old data won't change (they is how this feature is able to work without needing block-pointer rewrite). See this presentation: link to this video…

> AFAIK ZFS pools can't easily be resized > you generally have to recreate them Not anymore. There is RAIDZ Expansion available. Happens on the fly. - link to this video - link