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From: "Rodney W. Grimes" <rgrimes@GndRsh.aac.dev.com>
Message-Id: <199602011624.IAA20540@GndRsh.aac.dev.com>
Subject: Re: Am I root?! or How to mirror an 'a' partition?
To: stesin@elvisti.kiev.ua (Andrew V. Stesin)
Date: Thu, 1 Feb 1996 08:24:32 -0800 (PST)
Cc: hackers@freebsd.org, ccd@forgery.CS.Berkeley.EDU
In-Reply-To: <199602011355.PAA15508@office.elvisti.kiev.ua> from "Andrew V. Stesin" at Feb 1, 96 03:55:18 pm
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> 
> That was an announcement of new ccd driver and it's mirroring
> capabilities. I'd be _glad_ to test it, but I'm afraid it
> won't mirror an 'a' partition, and in case one disk will
> _totally_ go south late at night, it will wait for
> manual intervention anyway (i.e.: sd1 will become sd0 after
> panic -> 'sd1: device not configured' or whatever -> waiting
> for me to come and repair the box).
...

> 	dd if=/dev/rsd0[efg] of=/dev/rsd1[efg] bs=64k
> 
> work fine, but 
> 
> 	dd if=/dev/rsd0a of=/dev/rsd1a bs=64k
> 
> bombs and tells me: "dd: /dev/rsd1a: read-only filesystem"?
> sd0a is mounted on / and sd1a isn't mounted at all.

Because the first 16 blocks of the a partition are special, they
contain the bootblocks and disklabel.  These are protected from
writes and require a special ioctl to be able to write them.
> 
> 	dd if=/dev/rsd0a of=/dev/rsd1a bs=16b skip=1

Try adding a seek=1 so you skip over the first 16b of the output
device too.  Skip only effects the input file.


-- 
Rod Grimes                                      rgrimes@gndrsh.aac.dev.com
Accurate Automation Company                 Reliable computers for FreeBSD