When happens when Vdevs are different sizes

[Justin Aggus]

How does freeNAS handle the different sized Vdevs?

If I have 3 vdevs, 80Tb, 36Tb, and 30Tb will freenas write evenly to all 3, until the 30Tb fills up, and then write 1/3 slower?
Or will it fill them at a rate proportional to their size, so they all fill up together?

 

[mav@]

It should be something in between. It tries to write evenly on all vdevs, while possible, but if performance of some vdev drops (and it should drop first on smaller vdevs due to higher fragmentation), balance should change to bigger vdevs. But that is theory. In practice equal vdevs work better.

 

[Justin Aggus]

Thanks,

Does FreeNAS have a "Rebalance" command?
After running that command where will data end up?
(Say my System is 1/2 full)
40TB/80TB on vdev1, 18/36 on vdev2 and 15/30TB on vdev3

Or 24/80, 24/36, and 24/30

 

[mav@]

ZFS design does not allow such thing as rebalance. So no, it doesn't.

 

[Stux]

But if you mv content between datasets it will rewrite it...

 

[mav@]

But if you mv content between datasets it will rewrite it...

Yes, but if you have snapshots, you just double occupied space.

 

[Vito Reiter]

The only way you'd have vdevs of such capacity is to have multiple drives in each, so I'll assume you have more than 3 in each one. There's a sort of Read & Write threshold when it comes to RAID arrays which you have probably surpassed by several drives. I wouldn't expect much if any performance loss vin your zpool from a vdev filling up, although you should know it's highly recommended not to let them fill up passed 80%.