[afnog] FreeBSD mirroring

[Phil Regnauld]

On Fri, Apr 28, 2006 at 09:33:34AM +0100, Brian Candler wrote:
> > 	I haven't met any disks yet where the exact disk size, minus
> > 	the label offset, exactly matches a multiple of 512 bytes.
> 
> I think you mean "the exact disk size ... exactly matches a multiple of the
> cylinder size"

	Indeed.

	[...]

> So I think you're right; for legacy DOS-style machines you're probably safe
> enough, as long as you mirror the whole disk (/dev/da0) rather than just the
> FreeBSD slice (/dev/da0s1)

	Gmirror also allows you to mirror individual slices, or even
	partitions.  There you might have to be explicitly careful about not
	allocating the last megabyte or so, explicitly.

> For machines which have more modern partitioning systems (Macs?) this might
> not work. If you choose the so-called "dangerously dedicated mode", where
> the whole disk is used by FreeBSD with no partition table, then also you
> might be at risk.

	In principle, yes.

> Of course, if you create the partition as mirrored in the first place before
> you populate it then everything is fine whatever you do, but I'm not sure if
> the install process lets you do this at the moment.

	No, for no mirror or concatenation/raid system whatsoever (vinum, ccd,
	gmirror, ar*...).  It's a pain, but then again, sysinstall is very old :)