Mdadm on Electric Eel

Hi,

I’ve upgraded my TrueNAS SCALE installation to Electric Eel. I have a drive that I was using to import data that was from another Linux box that used mdadm mirroring. On Dragonfish I was able to utilise it by doing:

# mdadm --assemble /dev/md0 /dev/sdf1 /dev/sdg1

However, on 24.10, on running that I get:

modprobe: FATAL: Module md_mod not found in directory /lib/modules/6.6.44-production+truenas
mdadm: Fail to create md0 when using /sys/module/md_mod/parameters/new_array, fallback to creation via node
mdadm: unexpected failure opening /dev/md0

Has the required kernel support been removed? If so, was this deliberate or should I raise a bug for it?

Thanks!

Pretty sure it was deliberate. I don’t recall the exact specifics, but I think we ran into some cases where having it present was causing an issue with some certain setups when re-using disks from another system that had been previously formatted. Having it disabled fixed that so it didn’t have issues with pool creation IIRC.

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OK thanks for the info.

If it was auto-detecting and assembling RAID arrays I can see why that would be a problem. Although, if that was the case I would have thought there would be other ways to disable it without needing to remove the kernel functionality / module.

Removing all of the mdadm raid classes is annoying.

I personally use an old school raid1 for my swap space.

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There’s even a T3 video

To Swap or Not To Swap | TrueNAS Tech Talk (T3) E009

I’ve been running Linux systems for over 25 years and personally I always enable swap unless I’m running off a USB key or similar device.

Anyway - I want to understand why mdadm based raid support has been removed. Having it is useful for recovering data from other devices, as well as using SMR drives that zfs dislikes.

Try reaching out to Kris from the second post. It looks like it was removed due to other issues stated in the post.

It would be great to have it back as an option - especially if you’re trying to re-use some SMR drives.

For example I’ve a couple of 8TB SMR drives I’d like to use in an mdadm raid1 mirror as archive storage outside of my zfs pools.