On Wed, 2008-12-17 at 14:24 -0700, Andreas Dilger wrote:

I can't speak for btrfs, but I don't think multiple device access from
the filesystem is a "layering violation" as some people comment.  It
is
just a different type of layering.  With ZFS there is a distinct layer
that is handling the allocation, redundancy, and transactions (SPA,
DMU)
that is exporting an object interface, and the filesystem (ZPL, or
future
versions of Lustre) is built on top of that object interface.

Clean interfaces aren't really my best talent, but btrfs also layers
this out.  logical->physical mappings happen in a centralized function,
and all of the on disk structures use logical block numbers.

The only exception to that rule is the superblock offsets on the device.

-chris