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I have been a mdadm raid6 user for years and I'm very satisfied with it, but I would like to have a filesystem with snapshots, because fake snapshots on ext4 is eating all my diskspace.

I read here and there that btrfs is unsafe with raid 5/6, but I have the feeling the most often talk about some raid integrated in btrfs it self.

Is it safe to use btrfs over mdadm raid6 ?

I know an alternative option would be to use open-zfs, but I do not like this file system because it doesn't integrate cleanly inside the /dev/fstab structure.

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Yes, the problem with RAID5/6 is in the Btrfs implementation, using it with MD RAID should be safe. (If you stack btrfs on top of MD, the RAID logic happens in the block layer, filesystem doesn't even know it runs on a RAID.)

According to the btrfs wiki RAID5 and 6 is still not recommended to use and mkfs.btrfs now shows a warning when you try to use it, so it's a good idea to avoid it.

Btw. if you are already ok with stacking multiple layers, LVM would be another alternative here -- with LVM RAID for RAID and LVM thin provisioning for snapshots.

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  • I have no experience using LVM and in my experience, md raid6 has been pretty stable and safe over the years, so I'm considering adding it as a disk space allocation flexibility layer between my file systems and my md raid, but for flexibility only, because.I don't see how any snapshots at blocks level could be efficient in regard with disk space, compared to snapshots a file system level. Commented Feb 17, 2022 at 3:25
  • @Camion as I recall LVM raid is adopted from mdadm source, just integrated so you don't end up with 3-4 layer stacks of file system abstraction. So there is no reason to use LVM if you don't need the logical volumes (unless you just prefer the management syntax.). Commented Feb 13 at 15:17
  • LVM RAID shares the kernel RAID implementation with MD, the userspace implementation is separate. Commented Feb 13 at 16:38

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