DragonFly BSD
DragonFly kernel List (threaded) for 2006-01
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Re: cowloop technology

From: Ivan Voras <ivoras@xxxxxx>
Date: Wed, 11 Jan 2006 12:49:39 +0100

Thomas E. Zander wrote:

Well, it blocks raw device write if the reference count of this device
is not zero, i.e. you can't change the partition table of a disk in
multi-user mode if you have mounted filesystems on that disk.
It's a bit uncomfortable sometimes but I wouldn't go as far as to speak
of a particular problem. It reminds me that it's better to avoid some
things in multi-user mode :-)

As you said, it's better not to do it in multi-user mode, but it's actually a non-problem. AFAIK, a partition slicer could be implemented in a way that allows manipulating the partition table(s), but it would have to be done from inside the kernel (using the builtin userland <-> kernel communication facility for GEOM classes).

As for "cowloop", this is very similar what I did with "gjournal" (http://wikitest.freebsd.org/moin.cgi/gjournal). (note: this version gjournal is discintinued; future version(s) are in development).

GEOM is a *very* nice framework for this kind of thing.

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