I used to think this was a horrible layering violation as well. I'd been using various filesystems on LVM on RAID for a good 15 years, and from a certain point

I used to think this was a horrible layering violation as well. I'd been using various filesystems on LVM on RAID for a good 15 years, and from a certain point of view these layers make a great deal of sense. But these layers also impede a number of important things needed for data integrity checking and efficient rebuilding on failure, as well as complicating administration when you need to rebuild bits of an array after failure or fiddle with logical volumes. From the point of view of a sysadmin, ZFS is a revelation. Datasets are simple; creating, resizing (quota), deleting are trivial and safe. Operations on the pool are logical and (relatively) safer than with mdraid or hardware RAID. Snapshots are safer--you are doing it at the filesystem level rather than the block device level, so it can never be inconsistent or get corrupted when your snapshot device is invalidated at some arbitrary future point when the block delta exceeds the device size. The layering is still there. It's just subtly different. If you're used to LVM, then a "pool" is basically a volume group. The "vdev"s making up the pool are the physical volumes, which might be RAID sets. The "datasets" or "zvols" are logical volumes. It's a reinvention of what we already had, but it's more powerful, more flexible, and vastly easier to administer. I was sceptical, but after learning about it (and there is a learning curve), doing some simple test installs (often repeated as I realised I'd not set up the pool with the right blocksize or partition alignment, or the optimal dataset structure) and then some actual deployments, I'm a convert. What it offers is currently unmatched.