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    SubjectRAID4 with no striping mode request
    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.

    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.

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