Adding 2x8TB HDDs as Mirror to Pool, Efficient Way to Use Space

TrueNAS General
1

I didn’t know in which section to ask this question, and decided it might be most appropriate as a TrueNAS General category.

I’m sidegrading from CORE to SCALE 24.04.2.5, and adding a 2x8T mirror to my existing 2x8T mirror TANK pool (end result: 2 x 8T mirrors, ~16GB usable storage).

I’m at the stage of adding the new mirror, but … my existing 2x8T mirror is about 70% full, and the data is still there from when the HDDs were a CORE Pool. For context, this is a Plex Media Server.

To my understanding, if I just add the new mirror to the existing mirror, TrueNAS doesn’t do any “leveling” of the existing data across the two mirrors: the old data datasets and folders stay where they are on the original mirror, and new data is added to the remaining unused space and to the new mirror in an inscrutable manner.

My questions:

  1. Is there a performance hit or advantage with this arrangement (new data mostly stored to the new mirror)?

  2. Will I see any significant gain by going through the lengthy process of re-initializing the two mirrors to an empty state and then restoring my existing data onto the fresh pool from a backup? This should (?) distribute new data roughly equally across both mirrors.

I ask because the existing 2x8T mirror HDDs have been running for some time, whereas the new 2x8T mirror HDDs are burned in but not new. If I just add the new mirror to the pool and let new data land mostly on it, while the original mirror “coasts”, the entire pool of mirror HDDs should have a longer life - I think.

If I re-initialize the existing mirror and restore my backup onto the two mirrors from scratch “equally”, both mirrors experience roughly the same workload stresses under use - again, I think.

That is, if I just add the new mirror and go, am i setting myself up for a later “gotcha”? Or am I overthinking this? If the difference between the two ways of creating the expanded pool is negligible, then I’ll probably go the lazy route and let the new data land mostly on the new mirror.

Any inputs or suggestions welcomed. I’m not a TrueNAS newby, but don’t want to set myself up for a laer problem because I took the easy route.

2

There is supposedly a performance gain if the data is distributed more equally across both vdevs. If that is noticible in your use case depends on a lot of other factors.

You can “rebalance” the pool by recursively copying every $FILE to $FILE.new, deleting $FILE, renaming $FILE.new back to $FILE.

There are scripts to do that. Search the forum for “rebalance”.

HTH,
Patrick

3

There would likely be a small improvement to performance by rebalancing the data–though much of that would likely result from reduced fragmentation as well. FWIW, I don’t believe it’s worthwhile in the general case, and have never done any sort of “rebalancing” when adding vdevs to my main pool.

4

Thanks - that’s what I kinda suspected. I’ll check out the rebalance script.