Unable to create Log vdev pool

I have running truenas stable (TrueNAS-13.0-U6.1) , where I tried to create a mirrored log vdev pool with 2 RM-200 8GB Nvme drives as slog device for my nfs pool. I can see the drives and can create a standard data pool with this devices, but If I try to make a log pool (mirrored or stripped doesnt matter) the create button is not highlighted and I cant create it. Same with Cache metadata or Dedup pools.

Any help what I am doing wrong or is it a bug in the gui ?
Thanks for any help

Your terminology is confused, which makes it hard to follow exactly what you were trying to do in the GUI. You add vdevs to a pool.

Can you illustrate this with some screenshots, so we can follow along better?

Hi Eric, hi Tronic,
I’m new to the forum. A friend pre-install trueNAS on a spare PC.
I have a similar problem under TrueNAS-SCALE-23.10.2.
A main dataset was created, my two physical hard drives are visible and in mirror config but i have no pools yet and cannot create any as the UI asks for vDev but disks are used already.

Do I need to erase datasets (and/or credentials) in order to create pools?
Thank you for your help (redirect me if necessary)
Best regards

Your Pool is Home_NAS. If you have no further discs in your system, you cannot create another one.

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Welcome to the forums @tronic & @Loransea!

Both of you, @tronic & @Loransea, should review ZFS layouts. I don’t have a handy link to a Resource that describes ZFS pool layouts, so I will sum up some information.

All storage devices in TrueNAS are expected to use the whole storage device, (aka not sharing disk with other functions).

A ZFS Pool consists of at least 1 data vDev, (virtual device). Data vDevs can be single disk, Mirrored disks, (2 or more sub-Mirror disks), RAID-Zx, (similar to RAID-5/6) or dRAID, (too complex to talk about).

Their are optional vDevs that can be added in addition to data vDevs. For example, L2ARC, (Aka Cache), or SLOG, (Aka Log). These optional vDevs can be added later and in most cases be removable later, (but not all cases!).

Whence you have a ZFS Pool, you can create Datasets or zVols, (containers for foreign file systems, like for VMs). Datasets are where you would create shares for NFS or Samba.

ZFS is pretty different from other RAID methods, Volume manager schemes and file systems that without some ZFS knowledge, using TrueNAS can be painful. And in some cases, lead to less than optimally designed pools.

If you can describe you hardware and pool layout better, we might be able to assist further.

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Again, the terminology there is all wrong and very difficult to follow.

A ZFS pool is composed of vdevs (single disks, mirrors, RAIDZ or DRAID); it has one top-level dataset of the same name as the pool, and an arbitrary number of children of that dataset.

You do have a pool, Home_NAS, composed of a single pair of mirrored disks.

So what do you want to do next? Probably create some datasets for your data, right?


Hi Eric
here are a few screenshot, maybe now its more clear
This are my disks incl. the slog disks

and thats how I tried to create the log vdev, as you can see I cant press
create, looks lioke something is missing

It looks like you’re trying to create a new pool. What you want is something like Pool Header → Gear icon → Add vdevs

My fault. I tried to make a log vdev in a new pool without data vdev.

So I added them to the existing pool and now its working. I will do same tests in the next days if I get better performance with NFS and sync writes.

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Thanks for your clarification.
Well yes, then I would like to create «partitions» in my language or datasets for sharing data on our local network and also for cloud purposes so we could have a remote access to personnal and shared files.

I will get hopefully throught the process.

Is there some kind of map that could help me understand the wheres, whats and what-is-it-fors :face_with_head_bandage:

Last but not least, is it possible to «manage» that unique pool if not from storage where from?

Thank you and good weekend to you

Thank you Arwen for your help.
I’m on the stippy part of the learning curve I guess.
Will do some research about ZFS. If you know a link to a fairly clear and simple explaination, would you please share.
Have a good weekend.

So use the datasets tab.


See above.

Physical: Disks go into vdevs; vdevs go into pool.

Logical: Pool is sliced into datasets; sharing is applied to datasets.