Suggestions for the xen user's manual / wiki

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Suggestions for the xen user's manual / wiki

Michael Heyse
Hi,

there are two issues wich cost me some time to figure out and maybe it
made sense if someone put them into the documentation and / or wiki,
because without these addressed my system ran pretty instable and wasn't
very usable. This applies to xen-3.0 and kernel 2.6.12.6-xen:


1) Networking issue

With the default bridged setup, networking was quite flaky, e.g. samba
access from dom0 to a server in domU was impossible, ssh connections
froze, and dmesg contained lots of lines like "Received packet needs 8
bytes more headroom". A short explanation is found in

http://lists.xensource.com/archives/html/xen-devel/2005-12/msg00226.html

and is apparently due to some bug in the networking code. The suggested
workaround fixed this problem for me: set the mtu to 1400 for all
network interfaces in dom0 and domU.

I'm actually suprised that this is not a FAQ.


2) Scheduling issue

The system was unusable with high CPU load in dom0, all other domains
were almost dead, and there's apparantly no comprehensive documentation
of xen's schedulers (please point out the relevant docs to me if I'm
wrong, all I found was sedf_scheduler_mini-HOWTO.txt,
xenwiki/Scheduling and `man xm`, none of which helps to understand
scheduling well enough to change its parameters). Thanks to Tim Freeman
I found an easy setup that works for me: I set the scheduling parameters
for all domains via "xm sched-sedf <domID> 0 0 0 1 1" or something
similar corresponding to example "4 domains with weights 2:3:4:2" from
the sedf_scheduler_mini-HOWTO.txt with an additional 1 as the extratime
parameter if you want to allow a domain to allocate more CPU time if
other domains are idle.


With these two issues fixed the system runs just great.

As a total xen noob I don't dare touch the wiki, but if someone more
experienced could verify that these issues are valid, adding some hints
to the documentation could help other xen beginners.

Thanks,
Michael

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Re: Suggestions for the xen user's manual / wiki

Ralph Passgang
Am Dienstag, 10. Januar 2006 23:40 schrieb Michael Heyse:

> Hi,
>
> there are two issues wich cost me some time to figure out and maybe it
> made sense if someone put them into the documentation and / or wiki,
> because without these addressed my system ran pretty instable and wasn't
> very usable. This applies to xen-3.0 and kernel 2.6.12.6-xen:
>
>
> 1) Networking issue
>
> With the default bridged setup, networking was quite flaky, e.g. samba
> access from dom0 to a server in domU was impossible, ssh connections
> froze, and dmesg contained lots of lines like "Received packet needs 8
> bytes more headroom". A short explanation is found in
>
> http://lists.xensource.com/archives/html/xen-devel/2005-12/msg00226.html
>
> and is apparently due to some bug in the networking code. The suggested
> workaround fixed this problem for me: set the mtu to 1400 for all
> network interfaces in dom0 and domU.
>
> I'm actually suprised that this is not a FAQ.

I had exactly this problem on one testing box. Changing the mtu should help,
but better is to use xen-3.0-testing. This bug is fixed in xen-3.0-testing.

why it's not in the faq? probably because the most users won't see this on xen
3.0.0 release, just on some configuration this seems to happen.

and also this is not a long term issue and that's probably why not worth to be
mentioned in the faq.

--Ralph

[...]
> Thanks,
> Michael

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Re: Suggestions for the xen user's manual / wiki

Sipos Ferenc-2
Ralph Passgang wrote:
> I had exactly this problem on one testing box. Changing the mtu should help,
> but better is to use xen-3.0-testing. This bug is fixed in xen-3.0-testing.
> why it's not in the faq? probably because the most users won't see this on xen
> 3.0.0 release, just on some configuration this seems to happen.
> and also this is not a long term issue and that's probably why not worth to be
> mentioned in the faq.

I completely disagree with such an attitude. If a bug exists and there's
a known workaround to handle/get around it, it should definitely be
published in some sort of documentation/FAQ/whatever easily accessible
for masses. If I intend to run Xen in a production environment I'm not
about to use newly implemented, not comprehensively tested
functionalities (I assume, that's what the '-testing' tag is for) just
to avoid a glitch that normally can be handled with minimal effort.

So please someone having enough experience to update FAQ/Wiki, add it.

Thanks in advance.
Frank

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Re: Suggestions for the xen user's manual / wiki

Henning Sprang-2
In reply to this post by Michael Heyse
Michael Heyse wrote:
> Hi,
>
> there are two issues wich cost me some time to figure out and maybe it
> made sense if someone put them into the documentation and / or wiki,
>  

Can't you edit the wiki yourself? That's what wikis are there for.

(announcing here that you added some interesting information previously
not available might sometimes be a good idea...)

Henning

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Re: Suggestions for the xen user's manual / wiki

Michael Heyse
Henning Sprang wrote:
> Can't you edit the wiki yourself? That's what wikis are there for.

Sure I could, but I'm not even sure if these issues are real problems
that other people experience, too, or if I am the only one who sees
them, be it because of my hardware setup, inexperience with xen or
mental shortcomings. So if I get a "yes, I can confirm that these issues
are real problems" from someone who has a clue I'll gladly add them to
the wiki. Otherwise they are just confusing.

Thanks,
Michael

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Re: Suggestions for the xen user's manual / wiki

Karsten M. Self-3
In reply to this post by Michael Heyse
on Tue, Jan 10, 2006 at 11:40:54PM +0100, Michael Heyse ([hidden email]) wrote:

> Hi,
>
> there are two issues wich cost me some time to figure out and maybe it
> made sense if someone put them into the documentation and / or wiki,
> because without these addressed my system ran pretty instable and wasn't
> very usable. This applies to xen-3.0 and kernel 2.6.12.6-xen:
>
>
> 1) Networking issue
>
> With the default bridged setup, networking was quite flaky, e.g. samba
> access from dom0 to a server in domU was impossible, ssh connections
> froze, and dmesg contained lots of lines like "Received packet needs 8
> bytes more headroom". A short explanation is found in
>
> http://lists.xensource.com/archives/html/xen-devel/2005-12/msg00226.html
>
> and is apparently due to some bug in the networking code. The suggested
> workaround fixed this problem for me: set the mtu to 1400 for all
> network interfaces in dom0 and domU.
>
> I'm actually suprised that this is not a FAQ.

I've FAQd this:

    http://wiki.xensource.com/xenwiki/XenFaq#head-5e286e9bb5cf043f6534c917bc231af2af6adce2
 
 
> As a total xen noob I don't dare touch the wiki, but if someone more
> experienced could verify that these issues are valid, adding some hints
> to the documentation could help other xen beginners.

Wikis are designed to be self-service, to the extent possible.  Please
_do_ contribute and/or supply fixes or information as you feel
appropriate.


Cheers.

--
Karsten M. Self <[hidden email]>
XenSource, Inc.
2300 Geng Road #250                                +1 650.798.5900 x259
Palo Alto, CA 94303                                +1 650.493.1579 fax

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