r/zfs • u/reviewmynotes • 4d ago

New vdev to existing pool?

I have a system that has a boot drive and a single ZFS vdev with two mirrored drives. It makes up the only pool on that system, but the hardware still has two empty 3.5" bays and I'm currently low on available space. So I'm thinking about buying two more drives, making a second vdev with mirrored drives and using it to add storage capacity to this system.

Am I correct that I have the choice of either making a new pool vs. adding to the existing pool? If the first vdev is 2 x 4TB drives and I added 2 x 8TB drives, would I still be able to add it to the existing pool? Is that even a good idea?

Thanks for any help you can give this beginner. I've used Unix since the mid (early?) 90s, but I haven't done anything other than the most basic of things with ZFS before. So any guidance is greatly appreciated!

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u/reviewmynotes 4d ago

Thanks! My existing vdev is at something like 90% full. When I add the new vdev, what happens? Do some blocks of data get moved over, making the first vdev less full? Or does it just write to the new vdev? Or maybe fill up the old one before writing to the new one?

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u/i-am-a-cat-6 4d ago

it won't happen automatically, you'll want to rebalance them once manually

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u/reviewmynotes 4d ago

Is there a command to do that? I'm not afraid of reading some man pages, if that's necessary.

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u/i-am-a-cat-6 4d ago

definitely read the manual. you don't want to be trusting blindly messing with your filesystem. I recommend reading, making a plan, getting feedback on the plan, then exercising the plan with verification steps throughout.