Re: split root logical volume to var tmp home etc
On Tue, Jun 09, 2009 at 05:25:33PM -0500, Boyd Stephen Smith Jr. wrote:
> In <[🔎] 20090609211030.GA30776@m364d1.ece.northwestern.edu>, Zhengquan Zhang
> wrote:
> >On Mon, Jun 08, 2009 at 04:01:41PM -0500, Boyd Stephen Smith Jr. wrote:
> >> (0. [LVM specific] Create the new logical volume(s).)
> >> 1. Create new file system(s) on the block devices. [LVM: logical
> >> volumes]
> >> 2. Remount the original file system read-only.
> >> 3. Mount the new file system somewhere temporary.
> >> 4. Copy the data across.
> >> 5. Unmount the new file system.
> >> 6. Mount the new file system in it's permanent location.
> >> 7. Remount the old file system read-write.
> >> 8. Update /etc/fstab.
> >> (Here you may want to reboot to make sure the system will do so without
> >> manual intervention, to make sure your changes to the fstab are correct,
> >> and to make sure any of the files that have been moved the the new file
> >> system are no longer open on the old file system.)
> >>
> >> 9. Bind mount the old file system somewhere temporary.
> >> 10. Remove the data from the old file system via the bind mount.
> >> 11. Un- bind mount the old file system.
>
> Note that the numbers above exactly match the numbers below. Above is
> goals/semantics; below is actions/commands.
>
> >> Example: splitting / into / and /var:
> >> 2. mount -o remount,ro /
> >> (The above command probably won't work because some things are
> >> constantly writing to [e.g.] /var/log. You might try something like:
> >> fuser -mk / && mount -o remount,ro /. Or, you might just work from a
> >> system rescue disk.)
> >
> >I don't understand this. Why do I need to remount / read only?
>
> Because we are going to make a copy of (part of) it. If other programs make
> changes during the time it we are making our copy, our copy might be
> inconsistent or out-of-date. Therefore, we can't allow other programs to be
> writing changes until we are done getting our copy.
Thanks Boyd for another informative reply. Now I think I somewhat
understand now. Could you please confirm me on another question?
If I use a system rescue disk and do this with out the rsync command.
mount /dev/debian/var /var
the device /dev/debian/var is still empty and so we need the rsync step,
is it correct?
Thanks again for your time and patience,
Zhengquan
>
> You will need to make read-only whatever file system is being split.
>
> >> 3. mount /dev/debian/var /mnt
> >
> >Why do I need to mount the lv to mnt?
>
> This is a temporary location so that you can populate the new file system
> without disturbing the rest of the system.
>
> >> 4. rsync -HaAxX --progress --stats /. /mnt/.
> >
> >So everything in / will have a copy in /mnt ?
>
> Yes, but it won't descend into file systems mounted below /. In particular,
> /mnt doesn't contain a copy of /mnt! (An impossible situation, but that
> doesn't stop a computer from trying if you give it the right directions.)
>
> >and because we have mounted var lv to /mnt. var lv has a copy of
> >everything in / ?
>
> Ah, good catch. The correct command would be:
> rsync -HaAxX --progress --stats /var/. /mnt/.
>
> We only want the new lv to contain a copy of /var.
>
> >> 5. umount /mnt
> >
> >I don't understand this.
>
> Simply removes the new file system from its temporary mount point, since it
> is now ready to be used in its permanent location.
>
> >> 6. mount /dev/debian/var /var
> >
> >So now var lv is mounted to /var and it now has everything in /var?
>
> Yes.
>
> >> 7. mount -o remount,rw /
> >
> >Till here I am totally confused..
>
> This makes '/' read-write again. Note that '/var' was read-write as soon as
> you finished step 6, since it is now a separate file system. This command
> makes whatever else in is '/' read-write. For example, /root.
>
> >> 8. echo '/dev/debian/var /var ext3 relatime,acl 0 2' >> /etc/fstab
> >> (If you want: /sbin/shutdown -r now -t 5.)
> >> 9. mount -o bind / /mnt
> >> (Okay, so now /etc/fstab and /mnt/etc/fstab are the same file. However,
> >> /var/log/messages is on the new filesystem and /mnt/var/log/messages is
> >> on the old file system.)
> >> 10. rm -rf /mnt/* /mnt/.[!.]*
>
> This is definitely wrong, and would have made you very unhappy. The correct
> command is:
> rm -rf /mnt/var/* /mnt/var/.[!.]*
>
> This is removing the old version of /var that is part of the old file
> system. The current version is already on the new file system and in active
> use.
>
> >> 11. umount /mnt
> >
> >Sorry Boyd I don't quite understand, but thanks for your detailed reply
> >and I would greatly appreciate it if you can still help me to understand
> >it.
>
> HTH
> --
> Boyd Stephen Smith Jr. ,= ,-_-. =.
> bss@iguanasuicide.net ((_/)o o(\_))
> ICQ: 514984 YM/AIM: DaTwinkDaddy `-'(. .)`-'
> http://iguanasuicide.net/ \_/
>
--
Zhengquan
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