Extending a volume by adding enough disks to use RaidZ2

[Jevon Tane]

Hi all,

I have been trawling the forums and manuals but cannot find the clarification I need on this so my apologies if I have missed something.

I have 12 x 4TB disks in a RaidZ2 volume. I wish to add an additional 12 x 4TB disks to this volume to extend it and have noticed in Volume Manager when adding this amount of disks I get the option to choose a Volume layout e.g. Mirror, Stripe, RaidZ, RaidZ2 etc. My concern is this, I only want to extend the current volume using the existing RaidZ2 layout losing only the equivalent of 2 x 4TB disks of usable capacity to parity. If I add the 12 new disks and select RaidZ2 in the Volume layout before extending, will this create a new RaidZ2 layout in addition to the existing RaidZ2 which means I would now lose an additional 2 x 4TB disks of usable capacity to parity on top of what I'm already losing? That is would I now be losing the equivalent of 4 x 4TB disks of usable capacity to parity?

Any help would be much appreciated.

Kind regards,
Jevon.

 

[Ericloewe]

I have 12 x 4TB disks in a RaidZ2 volume

In a single vdev? That's wider than it should be.

I get the option to choose a Volume layout e.g. Mirror, Stripe, RaidZ, RaidZ2 etc. My concern is this, I only want to extend the current volume using the existing RaidZ2 layout losing only the equivalent of 2 x 4TB disks of usable capacity to parity. If I add the 12 new disks and select RaidZ2 in the Volume layout before extending, will this create a new RaidZ2 layout in addition to the existing RaidZ2 which means I would now lose an additional 2 x 4TB disks of usable capacity to parity on top of what I'm already losing? That is would I now be losing the equivalent of 4 x 4TB disks of usable capacity to parity?

You're missing one very important fact:
RAIDZ vdev configurations are immutable. You cannot add or remove drives from a RAIDZ vdev without destroying the pool.

Also, a 24-wide RAIDZ2 vdev would qualify as a genuine Very Bad Idea (tm). Not "meh, that's not recommended", but "guaranteed to cause all sorts of painful problems".

If you want 24 drives, I'd recommend three eight-wide RAIDZ2 vdevs.


Since all this is news to you, I highly recommend reading Cyberjock's guide, linked in my sig.

 

[Stux]

You can't increase the size of a vdev by adding disks, but you can add another vdev to a pool.

Luckily, what you want to do (a 24 way Raidz2) is so far outside of recommended, and what you can do (add a 12 way z2) is what you should do.

Mind you a 12 way z2 is actually slightly outside the recommended maximum number of disks in a z2 vdev too.

It would be better to use 3 8 way z2s, or two Z3s, but either way is 25% parity instead of 16% (or 8%!)

 

[Stux]

What eric said.

 

[Jevon Tane]

I consider myself schooled. Thanks for the advice guys and the recommended guide Ericloewe!

"Very Bad Idea (tm)" :D