Intel RST disk formats and cache drives.

[Zaphod Beeblebrox]

I see, and have used, the geom_raid bits to support Intel "software" RAID
systems, but the support is imperfect.

I have a laptop that has a rotating disk and an msata disk (for cache).  I
would like to use unused space on the msata disk for an installation of
FreeBSD.  However, FreeBSD doesn't see the "label" that uses the msata disk
as cache.

To try and figure things out more, I labelled an extra partition in windows
on the msata drive and then booted FreeBSD.  FreeBSD sees the extra
partition, but not the cache partition.  On further examination, it seems
the Intel scheme uses blocks at the "end" of the disk and presents windows
with a smaller disk.  This squares with the system "C" disk.  FreeBSD sees
the GPT partition as corrupt because the "backup" doesn't exist.  It does,
but it's not at the "end" of the disk as FreeBSD sees it.

So... when using the Cache, the intel driver in windows uses this labelling
scheme on both disks.

Has anyone looked at supporting this?  Would we modify the existing graid
to eat these disks, or some part of gpart/glabel?  It doesn't exactly fit
as there are also configurations where you have a RAID1 with a solid state
cache drive.  Is it possible we can support these configurations (at least
as labels) without supporting the cache semantics?