18TB on NAS326 as JBOD

The web interface doesn't let me format only 16TiB. If I select "Single volume on RAID" it tells me that it is too big.

What do you mean? It can create a 16TiB volume, but not as single volume on RAID?

Ah, I see.

I'm interested in the "better access performance"

For the record, I don't think you'll notice the difference.

I think you can create a disk group, and then from the command line just put an ext4 filesystem on the raw raid array. The 16TiB limitation is because ext4 originally had a limitation of 2^32 clusters. The limitation was lifted around kernel 3.2. Your box runs 3.10, so the kernel should support >16TiB. Don't know if mke2fs also supports that. Your original attempt to put ext4 on the disk was on the box?

To be able to put a filesystem on the raid array you'll first have to umount the volume(s) and maybe remove the logical volumes with lv/vgremove. That can be hard, as the firmware has internal bindmounts on the volume, and running processes which keep some files open. But there is a work-around, you can inject some code in the shutdown script to get shell access after the firmware has cleaned up it's mess. Search the forum for resize2fs, and on a NAS, not an NSA, which is slightly different.

When the volumegroup is gone, and an ext4 filesystem is put in it's place, I expect the firmware to use that, after a reboot.