Xen users - Apr 2008 - Newbie Question on Comparisons/Advice

Hi All,

I have been attempting to run WinXP under QEMU on a Fedora 8 laptop (HP
Pavilion dv8000).

It works and seems stable however anything needing the network is slow.
My need is to be able to demo some Win only applications that require a
remote connection.  Therefore this setup just won''t work.

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???   

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.  

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

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

Thanks,
Tim


 



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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.  :-)
:-)

HTH
-m

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On Monday 28 April 2008, Michael Jinks wrote:
> 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 have shrunk an grown ext3 partitions on LVM just fine. I have never had any 
problems with this, although I won''t imply this is without risk. You
should
always have a backup available before starting this operation !
> 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.
>
If you can afford to reinstall the system, you can probably also afford the 
risk involved in resizing an LVM. The good news is, fedora comes with a nice 
GUI interface that can do the resizing of both the ext3 partition as the 
logical volume it is stored in. This tool is called system-config-lvm or you 
find it in the menu under Administration - Logical Volume Management
(That''s
in the KDE menu''s anyway, I''m not sure where it''s put
under Gnome). It''s very
easy to use and until now it has never failed me.

Hope that helps,

Geert
-- 
Kobalt W.I.T.
Web & Information Technology
Brusselsesteenweg 152
1850 Grimbergen

Tel  : +32 479 339 655
Email: info@kobaltwit.be

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> 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.
LVM is generally recommended for Xen guest storage for reasons of performance, 
flexibility and robustness.  Basically, it''s about as fast as using raw
disk
but with the flexibility of being able to easily resize, snapshot, etc.  
Using files was sort-of not recommended for production environments since the 
Linux loopback device has some limitations (performance, scalability and 
flexibility, I think)

I''ve found that loopback devices are perfectly adequate for playing
around
with stuff on a small system, however.  Also, using tap:aio: instead of file: 
allows you to make a regular Linux file available as a VBD without the 
problems of the block loopback.  I think that''s the recommended way to
do
things these days.
> 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.
Indeed!
> > 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.
To be honest, if you have a Qemu hard disk image lying around from your 
previous experiments you *might* be able to simply point Xen at it using 
tap:aio: and have it boot.  Windows would probably cope with detecting the 
various changes in virtual hardware.  The major caveat here is that 
(depending on your Windows version / license) Windows can get difficult and 
decide that it''s been moved to another machine and wants to be
reactivated.
If you''ve got limited connectivity or a limited number of allowed 
activations, you''ll not want to do this.

Cheers,
Mark

-- 
Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/~maw48/pmpu/)

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