Optimizing vdev deployment in pool?

[bigzaj]

I have a newly built server that I am testing FreeNAS 10 (Corral) with. I currently have a zpool with one 6 drive vdev in Raidz2. These are WD Red 6TB drives. I am looking at expanding the pool with a second or third 6 drive vdev. I have an existing server I could steal 3TB drives from to setup a second vdev, but this won't be sufficient in terms of capacity and I would need to add a third in short order, likely WD Red 4TB based on the pricing. Result would be 18 drive pool in 3x6 drive vdevs.

Alternatively I have been considering biting the bullet on 6 new 6TB / 8TB drives and just added a second vdev. Setting aside the risk of more drives = more failure points, is there any performance differential between the two? Should I limit the number of drives or vdevs in a pool?

Thank you.

 

[SweetAndLow]

More vdevs will improve performance in the form of iops. You can have as many vdevs as you want.

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[Dice]

I think your reasoning pans out well ...ironically it is exactly the same drive sizes as my situation x)

Setting aside the risk of more drives = more failure points,

The key thing to keep in mind in this context and argument is that the safety of your data is a function of ZFS stripewidth and your propensity to replace failing drives within a vdev.
In other words, if you trust a 6x drive vdev raidz2 with your data, replacing failing drives quickly and successfully resilver - nothing changes if you add multiple vdevs to that pool. Resilvering is done on vdev level, not on pool level.

is there any performance differential between the two? Should I limit the number of drives or vdevs in a pool?

A few aspects factor into selecting stripe width, (number of drives within a vdev, raidz2).
For example, IOPS, resilvering time, space efficiency ....but probably most relevant to this (and many other situations) is the effect on the user's propensity to upgrade by adding another vdev consisting of same number of drives, prior to running out of space.
Running a 10drive Raidz2 sure maximizes space efficiency, but... once you want to upgrade, getting a batch of 10 drives is ....faaar steeper than getting 6 drives.