Re: [Xen-users] what's correct way of shrinking LVM based domU?

[Rudi Ahlers <Rudi@xxxxxxxxxxx>]

John Haxby wrote:
> Rudi Ahlers wrote:
> > ok, I don't know LVM at all. This is all a trail and error thing, 
> > since my previous installations have been image based and is 
> > difficult to resize.
> LVM isn't your problem :-)   Imagine you're shrinking the physical 
> disk in a real machine.  You need to make the file system and whatnot 
> in the real machine fit the new disk.
> > So, what you're saying is that I should do this from the domU itself? 
> > But that then means it need to be running?
> No, I'd be inclined to do all this from dom0 because you can handle 
> all the resizing stuff off-line.
> If you do "fdisk /dev/data/cpanel1" you'll see the disk layout that 
> your guest OS sees.  You can also see them with "kpartx 
> /dev/data/cpanel1" and kpartx's "-a" switch will create entries in 
> /dev/mapper.  The man page gives all the gory details.
> Typically, kpartx will report two partitions -- the first, smaller one 
> will be the guest's /boot, the larger one will be the guests logical 
> volume.
> So, and I haven't actually tried all of this together, but I think it 
> works.  So, make sure the domU is not running. "kpartx -va 
> /dev/data/cpanel1" (or whatever) will create new entries in 
> /dev/mapper for the disk image contained in that logical volume.  Now 
> run "vgscan" and that will look for any volume groups and, with any 
> luck will find and activate VolGroup00 which contains the file system 
> and swap partitions that the guest sees.
> > [root@HF-LINSRV01 ~]# xm console cPanel002
> > lvscan
> >  ACTIVE            '/dev/VolGroup00/LogVol00' [95.59 GB] inherit
> >  ACTIVE            '/dev/VolGroup00/LogVol01' [1.94 GB] inherit
> So this is what you'll see in domU now.
>
> At this stage, I'd be inclined to delete LogVol01 (which is almost 
> certainly swap, but you should check) and then shrink the file system 
> in LogVol00 with resize2fs.  Then shrink LogVol00 to snugly fit around 
> the file system, then pvresize to shrink the physical volume and the 
> lvcreate to re-create the swap partition.   So, broadly speaking, you 
> want to resize the file system to, say, 8GB, shrink the physical 
> volume to 10GB and then create a new 2GB logical volume for swap.  
> "vgdisplay -v /dev/VolGroup00" will tell you the name of the physical 
> volume you want to shrink.   Now you can de-activate VolGroup00, run 
> kpartx -vd to delete the entries in /dev/mapper and run fdisk on 
> /dev/data/cpanel1 and shrink the partition that holds the physical 
> volume.
> Finally, and this is the last step, you can run lvresize 
> /dev/data/cpanel1 to reduce the size of the logical volume in dom0.
> What you were doing was hoping that shrinking /dev/data/cpanel1 would 
> somehow shrink all of the data structures contained in that logical 
> volume.  LVM, of course, neither knows nor cares what you actually 
> store in a logical volume which is why it gives that warning message.
> I generally over-shrink file systems and whatnot and then grow them to 
> fill the available space once their container has been resized because 
> that means I don't need to worry about off-by-a-bit errors corrupting 
> vital file systems.
> Oh yes, back up anything important before you start :-)
>
> Hope this helps.
>
> jch
Sorry John, but what you're saying is very very confusing. I don't know 
LVM at all, and what I've done is purely by chance, and using the 
defaults when installing CentOS.
--

Kind Regards
Rudi Ahlers
CEO, SoftDux

Web:   http://www.SoftDux.com
Check out my technical blog, http://blog.softdux.com for Linux or other 
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