An alternative to booting to external media, etc, would be a bind
mount of / and /usr on separate temporary mount points, then dumping
the data between them, leaving the existing system chugging along. A
re-mount of the current /usr in -o ro mode might not be a terrible
idea in that case. I had a good bit of luck going that route. A simple
"cp -a" did the trick on the one system I've bothered with it on so
far. I have a couple laptops that're rushing headlong into unsupported
land right now though, so I'll be revisiting this soon enough.

As for specifics:

# Make the temporary working areas
mkdir /tmp/a; mkdir /tmp/b
# Make sure nothing changes in /usr while the copy is done
mount -o remount,ro /usr
# Mount a mirror of the source and destination filesystems
mount --bind /usr/ /tmp/a
mount --bind / /tmp/b
# And now, copy.
cd /tmp/a
cp -a ./ /tmp/b/usr/

The one big point of what not to do would be "mount --rbind". Very
important (recursive bind would have the current /usr still visible in
/tmp/b/usr/).

After all that, comment out /usr in fstab and reboot. You *could* even
just drop to a minimal runlevel that doesn't require /usr, unmount the
old one and then jump back to your standard runlevel, but due to the
reasons this is required now, I'm not entirely sure that option exists
anymore (i.e. too much is dependent on /usr).

That said, if you are booting to a LiveDVD --

On Mon, Dec 2, 2013 at 1:58 PM, Tanstaafl <tansta...@libertytrek.org> wrote:
> On 2013-12-02 1:47 PM, Thanasis <thana...@asyr.hopto.org> wrote:
>>
>> on 12/02/2013 04:02 PM Tanstaafl wrote the following:
>>>
>>>
>>> So, here's the plan, please check me...
>>>
>>> 1. Boot off of the latest gentoo LiveDVD
>>
>>
>> If you boot a different system to do the rsync, or, if you do it over
>> ssh, add the option --numeric-ids
>
>
> Thanks, but no, like I said, I'll just boot that system to a LiveDVD and do
> it from there...

That actually does fall under "boot a different system" since the
users won't line up between a LiveDVD and your actual system.

-- 
Poison [BLX]
Joshua M. Murphy

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