[possible] re-balance when adding a VDEV

[MtK]

Hey,
I have a pool with 2 x RaidZ vdevs, serving as a XenServer SR.
I'm about to add a 3rd vdev to it, among other things, to increase (at least a little bit) performance.

I know that adding the vdev, will not improve anything (at least anything significantly) for existing data, but only for NEW data being written. So I was thinking about creating new VDI for each VM, and move data to it. This way new data should be (re)written, therefor in a way re-balanced in the entire (3 vdev) pool.

Is that correct?

 

[Ericloewe]

As data gets rewritten, it will also be rebalanced (remember Copy on write, not Overwrite on write), so you might not gain much.

Generally, these maneuvers don't help much, long-term.

 

[MtK]

I am moving data, i.e from dataset1 to dataset2, so according to what you say, I am rebalancing.
Why won't I gain much?

 

[cyberjock]

It's not worth it. You could even argue that it's more risky and worse than leaving the data where it was due to fragmentation. In theoretical aspects, your performance will be faster. But in practical aspects, it won't matter at all. Nevermind the room for human error, etc. All you really are doing is forcing all the data you are copying to the new vdev. So you're balancing the total sum of data, but not the data itself.

Let the ZFS scheduler do its job and don't try to rebalance. You won't care in the end..

The only way to truly see a rebalance is to destroy the zpool and recreate it, then restore from backup. Everything else is just handwaving and claiming something that doesn't really matter. ;)