Xen users - Jul 2008 - questions regarding HVM and maximum block device size

Hello list,

right now I run a bunch of PVM Xen guests, everything is fine. But on the 
horizon, there a potential need shows up, that I may have to run one or more 
HVM guests.

Some time ago, I did some tests, and I observed the following on a host:
I activated the AMD VT extension in the BIOS, because I wanted to test to 
setup a HVM machine. While this was activated, the PVM domU running on the 
same host, had a unusual slow NFS performance. After I was ready with the 
tests, I disabled the AMD VT in the BIOS again, and the NFS speed 
was "normal" again. The NFS speed with VT enabled was about 1/3rd
slower
than without. The dom0 and domU are 64Bit, SLES10SP1 systems.
Is this normal what I''ve seen? If yes, I guess it''s not
recommended to run
PVM and HVM systems on the same dom0? Or if no, any idea, what I can do 
about it?

Further I''d like to know, whether a xm mem-set will work for HVM
domU''s?
I guess, in case the OS supports it, then it will work?
I''ve also read about paravirtual drivers for HVM guests, and
I''ve seen a
xen-balloon.ko for HVM Linux guests, but I want to run MS Windows, are there 
also such drivers available?

VMWare had, or still has, don''t use it anymore since there is xen ;), a
limit on the maximum size of a block device, at 2TB. So if I wanted to share 
a disk larger than 2TB, then the VMWare guest was/is only able to see the 
2TB but not more. Does in Xen exists a similar limit on block device size?

kind regards
Sebastian


_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

> right now I run a bunch of PVM Xen guests, everything is fine. But on the
> horizon, there a potential need shows up, that I may have to run one or
> more HVM guests.
>
> Some time ago, I did some tests, and I observed the following on a host:
> I activated the AMD VT extension in the BIOS, because I wanted to test to
> setup a HVM machine. While this was activated, the PVM domU running on the
> same host, had a unusual slow NFS performance. After I was ready with the
> tests, I disabled the AMD VT in the BIOS again, and the NFS speed
> was "normal" again. The NFS speed with VT enabled was about 1/3rd
slower
> than without. The dom0 and domU are 64Bit, SLES10SP1 systems.
> Is this normal what I''ve seen?
I don''t think that''s normal at all - it''s certainly
not the intended
behaviour!  You''re *just* running PV domains on the box, right?  The
only
difference is that you''ve enabled AMD-V in the bios?  That
shouldn''t make any
difference at all, so it''s most curious if there''s a
performance difference.

Have you also tried enquiring about this on SLES mailing lists / forums, in 
case it''s a SLES-specific problem?
> If yes, I guess it''s not recommended to run 
> PVM and HVM systems on the same dom0? Or if no, any idea, what I can do
> about it?
It should be fine to mix PV and HVM guests on the same system.  This is a 
pretty weird problem you''re seeing though - I''ve no idea what
would be
causing it.  Are you sure that the bios setting is the only thing that 
changed?  Have you double checked your measurements here?  I don''t mean
to
sound disbelieving, it''s just a very very strange problem to see!

Assuming this is definitely reproducible, further enquiries are the way 
forward.  Asking on the SLES support channels makes sense.  Asking on 
xen-devel may also be worthwhile.

Check xm dmesg and /var/log/xen/xend.log for any differences in output between 
the two cases.  I don''t know what I''d expect to see differ but
it''s worth a
try.
> Further I''d like to know, whether a xm mem-set will work for HVM
domU''s?
> I guess, in case the OS supports it, then it will work?
> I''ve also read about paravirtual drivers for HVM guests, and
I''ve seen a
> xen-balloon.ko for HVM Linux guests, but I want to run MS Windows, are
> there also such drivers available?
xm mem-set can work in principle for HVM domUs, yes.  AFAIK you won''t
be able
to grow a domain beyond its initial allocation at this point in time but you 
should be able to shrink and grow it within those bounds.

You need an appropriate driver for the HVM OS though.  As you''ve
noticed,
there is a Linux driver available.  For Windows, you''ll need to find
some
PV-on-HVM drivers for your platform.  I seem to recall Novell providing a 
driver pack for Windows on SLES - maybe you could look into that?  But 
there''s also a free set of PV-on-HVM drivers, with the development
being led
by James Harper although I don''t know if these have a balloon driver at
this
time...?  These are still in development, so they may not be recommended for 
use on a system containing important data or requiring high uptimes.  That 
said, I get the impression quite a few people are using them successfully 
having worked out any local problems.  Make sure to read through some mailing 
list archives on the drivers so you can learn of possible problems and 
actions to take to avoid them!

You may well want to experiment with PV-on-HVM anyhow to get better Windows IO 
performance.
> VMWare had, or still has, don''t use it anymore since there is xen
;), a
> limit on the maximum size of a block device, at 2TB. So if I wanted to
> share a disk larger than 2TB, then the VMWare guest was/is only able to see
> the 2TB but not more. Does in Xen exists a similar limit on block device
> size?
I think there is a maximum block device size under Xen but I''m not sure
what
it is.  If you search the mailing list archives you may find some useful 
information on this.

Cheers,
Mark

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

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

http://forums.opensuse.org/applications/389319-how-create-pv-centos-5-domu-opensuse-11-dom0.html#post1836391



      

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

Hi Mark,

Mark Williamson <mark.williamson@cl.cam.ac.uk> wrote:
> > right now I run a bunch of PVM Xen guests, everything is fine. But on 
the
> > horizon, there a potential need shows up, that I may have to run one
or
> > more HVM guests.
> >
> > Some time ago, I did some tests, and I observed the following on a
host:
> > I activated the AMD VT extension in the BIOS, because I wanted to test
to
> > setup a HVM machine. While this was activated, the PVM domU running on
the
> > same host, had a unusual slow NFS performance. After I was ready with 
the
> > tests, I disabled the AMD VT in the BIOS again, and the NFS speed
> > was "normal" again. The NFS speed with VT enabled was about
1/3rd slower
> > than without. The dom0 and domU are 64Bit, SLES10SP1 systems.
> > Is this normal what I''ve seen?
> 
> I don''t think that''s normal at all - it''s
certainly not the intended
> behaviour!  You''re *just* running PV domains on the box, right? 
The only
> difference is that you''ve enabled AMD-V in the bios?  That
shouldn''t make
any 
> difference at all, so it''s most curious if there''s a
performance
difference.
> 
> Have you also tried enquiring about this on SLES mailing lists / forums, 
in 
> case it''s a SLES-specific problem?
> 
> > If yes, I guess it''s not recommended to run 
> > PVM and HVM systems on the same dom0? Or if no, any idea, what I can
do
> > about it?
> 
> It should be fine to mix PV and HVM guests on the same system.  This is a 
> pretty weird problem you''re seeing though - I''ve no idea
what would be
> causing it.  Are you sure that the bios setting is the only thing that 
> changed?  Have you double checked your measurements here?  I don''t
mean to
> sound disbelieving, it''s just a very very strange problem to see!
> 
> Assuming this is definitely reproducible, further enquiries are the way 
> forward.  Asking on the SLES support channels makes sense.  Asking on 
> xen-devel may also be worthwhile.
> 
> Check xm dmesg and /var/log/xen/xend.log for any differences in output 
between 
> the two cases.  I don''t know what I''d expect to see
differ but it''s worth
a 
> try.
thank you for these comments, right now I do not have spare hardware 
available to make some new tests. But what you say lets make me hope that I 
either oberserved sth. wrong, or this was specifically to that machine where 
I observed it. I''ll retest when I get the new box for the HVM machine,
and
will ask on the -dev and SLES list, if I see the behavior again.
> 
> > Further I''d like to know, whether a xm mem-set will work for
HVM domU''s?
> > I guess, in case the OS supports it, then it will work?
> > I''ve also read about paravirtual drivers for HVM guests, and
I''ve seen a
> > xen-balloon.ko for HVM Linux guests, but I want to run MS Windows, are
> > there also such drivers available?
> 
> xm mem-set can work in principle for HVM domUs, yes.  AFAIK you
won''t be
able 
> to grow a domain beyond its initial allocation at this point in time but 
you 
> should be able to shrink and grow it within those bounds.
> 
> You need an appropriate driver for the HVM OS though.  As you''ve
noticed,
> there is a Linux driver available.  For Windows, you''ll need to
find some
> PV-on-HVM drivers for your platform.  I seem to recall Novell providing a 
> driver pack for Windows on SLES - maybe you could look into that?  But 
> there''s also a free set of PV-on-HVM drivers, with the development
being
led 
> by James Harper although I don''t know if these have a balloon
driver at
this 
> time...?  These are still in development, so they may not be recommended 
for 
> use on a system containing important data or requiring high uptimes.  That 
> said, I get the impression quite a few people are using them successfully 
> having worked out any local problems.  Make sure to read through some 
mailing 
> list archives on the drivers so you can learn of possible problems and 
> actions to take to avoid them!
> 
> You may well want to experiment with PV-on-HVM anyhow to get better 
Windows IO 
> performance.
> 
> > VMWare had, or still has, don''t use it anymore since there is
xen ;), a
> > limit on the maximum size of a block device, at 2TB. So if I wanted to
> > share a disk larger than 2TB, then the VMWare guest was/is only able
to
see
> > the 2TB but not more. Does in Xen exists a similar limit on block
device
> > size?
> 
> I think there is a maximum block device size under Xen but I''m not
sure
what 
> it is.  If you search the mailing list archives you may find some useful 
> information on this.
Well, I did, but maybe not with the right keywords, or maybe not intensive 
enough, however, will take a look again.


thanks a lot
Sebastian


_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users