Adding a vdev to a pool

[Fritzolio]

Are there any best practices when adding vdevs to an established pool? Does adding vdevs affect the redundancy of the existing vdev?

I searched but couldn't find the answers. A link to the pertinent info would be lovely.

Thanks :)

 

[Bidule0hm]

I recommend to read Cyberjock's ZFS Guide (link is in my signature) ;)

But in short you should use the same RAID type as the vdev(s) already in the pool (if you use RAID-Z2 then the new vdev should be RAID-Z2 too) with the same number of drives. However you can use drives of a different size from the size of the drives already in the pool (for example 6 TB for the new drives when there's 4 TB in the pool).

It doesn't affect the redundancy (reliability is a better word here I guess) of the first vdev but it lower a bit the reliability of the pool (of course as you add things there's more things than can die than before), however you shouldn't be worried about that, it's marginal.

 

[DrKK]

you should use the same RAID type as the vdev(s) already in the pool

Really? Why should that be? In principle this is not necessary, right?

 

[Bidule0hm]

No, but it's recommended.

 

[DrKK]

No, but it's recommended.

I got *THAT* part sir. I am asking WHY it's recommended.

 

[BigDave]

I got *THAT* part sir. I am asking WHY it's recommended.

Careful, he's testing you...

 

[Bidule0hm]

Because if you put a mirror with a RAID-Z2 for example the mirror will kill the reliability relatively to the RAID-Z2. I suspect there's also deeper ZFS concerns about doing that, maybe @cyberjock can enlight us on this.

@BigDave lawl :D

 

[DrKK]

Oh well, I assume that the person putting in the second vdev into the same pool already understands those exigencies; i.e., that their pool integrity is capped at the minimal integrity in any given vdev.

But other than that?

I'm not testing him. I thought we might be talking about something I didn't know, so I'm seeking enlightenment.

 

[BigDave]

I was just trying to be funny about the testing thing, sorry
What little I know about Vdev performance (it's very little)
is basically a read the same way as you've written kinda thing.
I'm sure I'm gonna get schooled here, cause this is a wag ;)

 

[solarisguy]

I understood that the new pool would be getting the worst of each vdev type.

 

[Ericloewe]

I got *THAT* part sir. I am asking WHY it's recommended.

Cyberjock made a cryptic comment the other day about not mixing RAIDZ levels and/or mirrors.

I'm still hoping for a followup on that... I don't really see why it would matter, reliability issues and silly mixtures aside.

 

[Fritzolio]

Thanks all. My understanding of FreeNAS is growing every day. :)