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My stack at the moment is 6x [10 TB WD Red Pros -> dm-integrity] -> mdadm raid6 -> dm-crypt -> LVM. One of my LVM volumes is btrfs. I only use LVM because I've had terrible performance for VM images stored in btrfs volumes. You can always use fallocate and disable CoW for the image but that prevents taking snapshots.

I'd if there was a zvol equivalent in btrfs.

Neither fallocate nor nodatacow will prevent taking snapshots. The result of writing to nodatacow shared extents is that the write must be COW, not an overwrite. Same as a reflink copy of a file on XFS. New "overwrites" are COW, subsequent overwrites are in fact overwrites.

We've phasing out using btrfs for RAID at work. It has been just a lot more straight-forward and easier to manage MD for the RAID-1 on our servers. We still use btrfs for container filesystems, subvolume snapshots are so handy. But that is btrfs on top of a MD RAID array.

Oh I'll get right on downloading that to my FreeBSD storage boxes, oh wait a minute.....