is there a COW inside the kernel ?

["roland" <devzero@web.de>]

hello !

is there an equivalent of something like

cowloop ( http://www.atconsultancy.nl/cowloop/total.html ) or md based cow 
device ( http://www.cl.cam.ac.uk/users/br260/doc/report.pdf ),

i.e. a feature called "Copy On Write Blockdevice" inside the current or the 
near-future mainline kernel (besides UserModeLinux Arch)?

i'm not sure - i think i remember having read that something like this can 
be probably done , but i don`t remember anymore what it was.

i would find this useful for several purpose, but i don`t want to patch my 
system with 3rd party drivers or "non-standard" stuff -  or even recompile 
the kernel.

can someone help out with some information ?

TIA

roland k.
system engineer