RFC: backporting GEOM to the 4.x branch

[Matthew Dillon]

:As far as BUF/BIO model changes are concerned, I understand that the I/O subsystem
:was supposed to be moved from KVA mapped buffers to page lists (using XIO to
:manage the page lists). Have any other significant changes been made to the FreeBSD
:BUF/BIO model that I should be aware of to be able to integrate deGEOMified Disk
:Encryption (dGDE) into DragonFly BSD?
:
:ALeine

    The current BUF/BIO model is very close to FreeBSD-4.  We haven't done any
    of the truely major bits of work on it yet (like changing the block
    numbers to 64 bit byte offsets and things of that ilk).

    The only big difference relative to FreeBSD-4 is that the actual I/O
    sequencing has changed slightly, and the device representing a labeled
    disk is no longer overloaded onto the device reprsenting a 'raw' disk.
    That is, they are distinct devices.  This means that implementing a
    'raw' block device should be fairly easy because you don't have to
    worry about the disklabel recursion mess.  This is what I meant by the
    block devices being fully recursable in DragonFly.  In 4.x the b_driver
    fields are used to overload meaning onto the raw disk device which makes
    creating stacks of block devices with different functions (e.g. encrypt,
    raid, network interface) difficult.  That's all been cleaned up in
    DFly.

					-Matt
					Matthew Dillon 
					<dillon at xxxxxxxxxxxxx>