Re: cgd gives read-only error

[Julian Coleman <jdc%coris.org.uk@localhost>]

Hi,

> Although you can dd zeros over all rraid1d, I expect you can't dd
> zeros all over rraid1a.  The disk drivers refuse writes through
> non-raw partitions to the sector where the disklabel lives, so if
> raid1a covers raid1's disklabel, and you configure cgd0 on raid1a,
> then raid1a will refuse such a write.

As pointed out, the error occurs because the extent of the cgd covers the
disklabel of the underlying partition.  I think that the reason that the
error is non-intuitive is because it's possible to create a raid disk on
(e.g.) sd0a, where sd0a covers the whole of the physical disk and not
see an error when writing to any part of the raid disk.  Try this with a
cgd disk, and you will see the read-only error, so you have to move the
start of sd0a manually.

> None of it makes much sense to me.  Why exactly are we "scrubbing"
> the disk before setting up the cgd?  Just fill the cgd with 0 and
> you are filling the disk with what, really, had darned well better
> look like random data.

See chapter 14 of the guide:

  http://www.netbsd.org/docs/guide/en/chap-cgd.html#chap-cgd-example

where we create a cgd with a random key and use that to fill the disk with
random date.  (I agree that we could just as easily do this on the real
cgd.)

Thanks,

J

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