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"Kyle" == Kyle Sanderson [off-list ref] writes:

hi DM and Linux-RAID,
There have been multiple proprietary solutions (some nearly 20 years
old now) with a number of (userspace) bugs that are becoming untenable
for me as an end user. Basically how they work is a closed MD module
(typically administered through DM) that uses RAID4 for a dedicated
parity disk across multiple other disks.

You need to explain what you want in *much* beter detail.  Give simple
concrete examples.  From the sound of it, you want RAID6 but with
RAID4 dedicated Parity so you can spin down some of the data disks in
the array?  But if need be, spin up idle disks to recover data if you
lose an active disk?  

Really hard to understand what exactly you're looking for here.

As there is no striping, the maximum size of the protected data is the
size of the parity disk (so a set of 4+8+12+16 disks can be protected
by a single dedicated 16 disk).When a block is written on any disk,
the parity bit is read from the parity disk again, and updated
depending on the existing + new bit value (so writing disk + parity
disk spun up). Additionally, if enough disks are already spun up, the
parity information can be recalculated from all of the spinning disks,
resulting in a single write to the parity disk (without a read on the
parity, doubling throughput). Finally any of the data disks can be
moved around within the array without impacting parity as the layout
has not changed. I don't necessarily need all of these features, just
the ability to remove a disk and still access the data that was on
there by spinning up every other disk until the rebuild is complete is
important.

The benefit of this can be the data disks are all zoned, and you can
have a fast parity disk and still maintain excellent performance in
the array (limited only by the speed of the disk in question +
parity). Additionally, should 2 disks fail, you've either lost the
parity and data disk, or 2 data disks with the parity and other disks
not lost.

I was reading through the DM and MD code and it looks like everything
may already be there to do this, just needs (significant) stubs to be
added to support this mode (or new code). Snapraid is a friendly (and
respectable) implementation of this. Unraid and Synology SHR compete
in this space, as well as other NAS and enterprise SAN providers.

Kyle.