This may be a naive question since I'm new to this and I cannot find any results about mdadm+zfs, but after some testing it seems it might work:

The use case is a server with RAID6 for some data that is backed-up somewhat infrequently. I think I'm well served by any of ZFS or RAID6. Platform is Linux. Performance is secondary. So the two setups I am considering are:

• A RAID6 array plus regular LVM and ext4
• A RAID6 array plus ZFS (without redundancy). Is this second option that I don't see discussed at all.

Why ZFS+RAID6? It's mainly because the inability of ZFS to grow a raidz2 with new disks. You can replace disks with larger ones, I know, but not add another disk. You can accomplish 2-disk redundancy and ZFS disk growth using mdadm as the redundancy layer.

Besides that main point (otherwise I could go directly to raidz2 without RAID under it), these are the pros-cons that I see for each option:

• ZFS has snapshots without preallocated space. LVM requires preallocation (might be no longer true).
• ZFS has checksumming (very interested in this) and compression (nice bonus).
• LVM has online filesystem growth (ZFS can do it offline with export/mdadm --grow/import).
• LVM has encryption (ZFS-on-Linux has not). This is the only major con of this combo I see. I guess I could go RAID6+LVM+ZFS... seems too heavy, or not?

So, to close with a proper question:

1) Is there anything that inherently discourages or precludes RAID6+ZFS? Anyone has experience with a setup like this?

2) Are there possibilities for checksumming and compression that would make ZFS unnecessary (maintaining the possibility of filesystem growth)? Because the RAID6+LVM combo seems the sanctioned, tested way.