Re: [9fans] now for something completely devfs :)

[Geoff Collyer <geoff@collyer.net>]

I think the paragraph on the wiki would be clearer like this:

	If you already have a fossil and want to add another disk to mirror
	it, you can setup the mirror as said before, with the original fossil
	device (possibly a partition) being mirrored by the new disk, start
	your fossil and while you are using it, use dd(1) to copy the fossil
	device to the mirror device or partition.  Since writes go to all
	devices in the mirror but reads are performed just by the first device
	(as long as it has no read errors), the copy can be made this way with
	relative safety.  Races are still possible, since dd might read a
	block from the fossil device, fossil might change it and write it to
	both devices (original fossil and mirror) and then dd might write the
	now-stale block to the mirror, so it's safer to make the copy when
	fossil (and venti, if you mirror it) are idle.  This might require
	booting your file server off another standalone Plan 9 file & cpu
	server.

How do you know if a disk in a mirror has died?  The kernel should
print I/O-error messages on the console.  devfs should cope until you
can shut the system down and replace the dying disk.  With modern
disks, I/O errors are generally a sign of degradation: the disk has
forwarded as many bad sectors as it can and now the reserved bad
sector areas are full.