Re: [Xen-users] Getting started with Xen

[Ralph Passgang <ralph@xxxxxxxxxxxxx>]

Am Samstag, 28. Januar 2006 15:02 schrieb Jean-Michel Hiver:
> > Yes it is. I would recommend the Sarge packages at
> >
> > deb http://packages.debianbase.de/sarge/i386/xen3 ./
> >
> >> Is it possible to "migrate" a virtual machine to a completely
> >> different PC (with different hardware specs)? That would be a very
> >> useful feature to have for upgradability.
> >
> > Yes. Because Xen completely hides the hardware you can move one or
> > several server to another physical server. You just turn the server
> > off and use your favorite tool (rsync, dump/restore or dd) to copy the
> > files within the virtual server or the whole filesystem(s). If you go
> > nuts and by a SAN for your servers you can even do this process
> > without halting the servers.
>
> Wow! This sounds very cool.
>
> If Xen really *does* completely hide the underlying hardware, so that
> virtual OSes running have no "way" of knowing what their hardware is, I
> really see it as a big plus.

not 100% completely... you will see the real cpu type in /proc/cpuinfo and 
your xen domain will use all available cpu command sets / features (like sse, 
mmx, ...) that the real physical cpu offers.

This is only a problem for live/no-downtime migration, because if you migrate 
a xen domain from a cpu with more features to a cpu with less features (or at 
least not the same features) then your xen domain will reboot immidently 
after the migration process. For the other way, migration a xen domain from a 
cpu with less features to a better cpu is not a problem. Your xen domain just 
will not use the features of the better cpu as long as the domain doesn't get 
restarted.

But live migration is only possible if you have a san-like solution, because 
all xen hosts that are involved in the migration has to have access to the 
same blockdevices at the same location in /dev. If you only want to migrate 
xen domains from one xen host to another (or even from a real server to a xen 
system) then you will be fine anyway, because then you don't need a san or 
the same cpu specs on all machines.

Migration from a real host to a xen system is quite easy. stop the real 
machine, start it with knoppix or another minimal system where you have some 
tools available and copy all files (with tar/scp or rsync or ...) to your xen 
host and copy also the required kernel modules and move /lib/tls away. After 
that you can start the system again... this time virtualized with xen...

with a bit practise you will be able to move real system to xen system in 
<5-10 min + the time that is needed for copying the files from the real host 
to the block device (or loop device) on your xen host.

> I think I can tolerate about 1 hour of downtime per server if the work
> is done at night, so I guess migration won't be a big issue then.

it depend on the size of your servers (amout of data that has to be copied) 
but I guess you will be faster then 1h/server on normal server systems 
(webserver, mailserver, etc.).

Normale xen is ideal for small to medium sized server, because for the real 
big ones real physical hardware is better.

we have running 13 virtual servers on one dual xeon cpu system with 4GB ram 
and 300GB HDD (sata + raid5). the cpu load is still ok, but the hdd activity 
is quite high, so I think about 15-16 virtual system will be the max for us 
on this system... our customers are all very happy with their virtual 
servers, no real problems for now.

xen definitly rocks :)

>
> Cheers,
> Jean-Michel.

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