Re: [Xen-users] IBM x445, anyone using it?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> <mark.williamson@xxxxxxxxxxxx> wrote:
> > Nb VMware would give you full virtualisation support on your pre-HVM
> > hardware, which Xen can't give you.
>
> Good point...  but isn't there really a difference between say, full
> HVM virtualization on Xen and VMWare which is more emulation in this
> case?  That's just for curiosity sake...

Well, interesting question...

It's implemented differently but in terms of guest behaviour I don't think 
you'll notice any difference in real use.  Technically hardware support 
virtualisation might be more complete but VMware have, at this point, had 
many years to get really good at doing convincing full virtualisation in 
software.  They actually recompile guest machine code at runtime in order to 
reduce emulation overheads, whilst hiding this from the guest.

VMware is quite highly optimised to run guests efficiently without hardware 
support.  ISTR them saying when HVM hardware first came out that they'd 
benchmarked and found their software-only solution was faster than their 
HVM-based solution, so they were sticking with the software solution for the 
time being.  That doesn't tell you much about the performance of 
HVM-vs-software solutions in general, though, and both the hardware and the 
software has moved on since then.

What it does tell you is that that HVM isn't automatically "better" than a 
pure software solution, although it is likely to leave the latter behind as 
time progresses.  Using VMware to do full virtualisation on old non-HVM 
hardware is not going to be especially fast but it may well be quite usable.  
Equally well, using Xen to run supported PV guests is likely to yield much 
better performance on the same hardware, if you have the option of running 
supported guests in the first place (i.e. don't want Windows etc).

> >> Xen, on the other hand, works rather well with RHEL 5 and SLES 10, and
> >> I've heard, though not tried this myself, RHEL 4 and SLES 9.
> >
> > What you're running into is most to be a weird CentOS / virt-manager
> > behaviour rather than a true Xen problem or a hardware-specific problem.
>
> I think it has to do with those pre-configured images that you can
> download via the internet.  I've never had luck getting them to work
> correctly, and it usually has something to do with the filesystem.
> I've had no real problem, for instance, building a fresh domU on
> RHEL/Xen, then tarring that up and saving it to deploy on another
> RHEL/Xen box, but as far as using one of the pre-made
> CentOS/Fedora/Slackware/distroOfchoice images, I've had no luck at
> all.
>
> Granted, I only run the Xen versions that ship with RHEL and SuSE, so
> I dont do a lot with other Xen versions...

I've had great success with letting virt-manager do a network install of PV 
CentOS/Fedora guests over HTTP from an RPM repository somewhere.  I think 
I've done it both by serving a local RPM repository using Apache and by 
connecting to public mirror sites.  Works great for me and is so convenient 
I've never felt the need to use preconfigured images.  I do sometimes clone 
my own images in order to save the install time, though.

Cheers,
Mark

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

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