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xen-users
Re: [Xen-users] what's correct way of shrinking LVM based domU?
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
technical stuff, or visit http://www.WebHostingTalk.co.za for Web Hosting stuff
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