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Re: [Xen-users] Newbie Question on Comparisons/Advice

To: Tim Cook <timothywayne.cook@xxxxxxxxx>
Subject: Re: [Xen-users] Newbie Question on Comparisons/Advice
From: Michael Jinks <mjinks@xxxxxxxxxxxx>
Date: Mon, 28 Apr 2008 12:03:10 -0500
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On Mon, Apr 28, 2008 at 01:38:01PM -0300, Tim Cook wrote:
> I have been looking at XEN the past few days and trying to digest all
> the great documentation.  It seems to me that I would want Windows to
> run in an LVM VBD environment???   

You could, but given the risks of shrinking a filesystem (I've done it
successfully with ReiserFS, once, just to see if it worked; never tried
with any filesystem that had seen serious use and never with ext[23])
you might as well make a disk file within your existing filesystem.  If
you've been reading Xen docs you've probably run across examples for how
to do that.

I've read (have not done comparisons for myself) that a loopback-mounted
file in a Linux system will give comparable performance to a raw device
(i.e. LVM volume).  The advantage of LVM is in the flexibility, but
you're in a situation where the up-front costs (shrinking filesystem,
spending some time with the LVM docs) might outweigh the advantages.

> I've been using Linux at various levels since 1994.  BUT! I haven't kept
> up with technical stuff lately and really have only a vague clue about
> LVM.  When I installed Fedora I just accepted the defaults which created
> just one logical volume.  Now after reading the XEN docs I think that
> there will be a need to have two LV's?  Then reading the LVM docs it
> seems that this will require reducing the filesystem (ext3) size before
> reducing the LV size thus putting my data at risk.  

If you want to do it that way, yeah, you'll need to shrink the
filesystem, then you'll need to shrink the (virtual) device it lives on,
and then you can create a new virtual device in the freed space.  But
it's quicker just to dd a bunch of zeroes into a file and treat that as
a disk.

> This now brings me back to having to start over reinstalling Fedora;
> from my understanding.

Well, not necessarily, unless the filesystem shrinking damages your
installation, but given the possibility of that I'd go for a loopback
file if it were me.

> I hope I have missed something and made this much more difficult than it
> really is.  :-)



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