Xen users - Apr 2008 - How to stop yum update on CentOS 5.1 to modify grub

Hi all

I just got a shocker today on a CentOS 5.1 x64 Xen server where the xen 
VPS didn''t work properly so I thought I''d reboot it, but upon
reboot it
gave me some memory errors, so I rebooted the server and couldn''t start
Xen at all. Then after poking around I thought I''d check to see if the 
Xen kernel is running, and it''s not running. It seem yum updated has 
updated the kernel, and then updated grub to use the new CentOS kernel, 
which is what it''s supposed todo. But how do I stop yum from doing it?

-- 

Kind Regards
Rudi Ahlers
CEO, SoftDux
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Rudi Ahlers schrieb:
> Hi all
> 
> I just got a shocker today on a CentOS 5.1 x64 Xen server where the xen 
> VPS didn''t work properly so I thought I''d reboot it, but
upon reboot it
> gave me some memory errors, so I rebooted the server and couldn''t
start
> Xen at all. Then after poking around I thought I''d check to see if
the
> Xen kernel is running, and it''s not running. It seem yum updated
has
> updated the kernel, and then updated grub to use the new CentOS kernel, 
> which is what it''s supposed todo. But how do I stop yum from doing
it?
I wouldn''t hack into packet managers. It''s better to use
menu.lst itself to
reflect the changes you want.

I don''t know which version of grub centos uses, but on recent
debian/ubuntu
systems, there are commented lines which are used for default values.

e.g.
## Xen hypervisor options to use with the default Xen boot option
# xenhopt=vtd=1

## Xen Linux kernel options to use with the default Xen boot option
# xenkopt=console=tty0 swiotlb=128 max_loop=64

and

## If you want special options for specific kernels use kopt_x_y_z
## where x.y.z is kernel version. Minor versions can be omitted.
## e.g. kopt=root=/dev/hda1 ro
##      kopt_2_6_8=root=/dev/hdc1 ro
##      kopt_2_6_8_2_686=root=/dev/hdc2 ro
# kopt=root=UUID=2c9954c1-d8f5-477a-a808-39f768b10ac3 ro

## additional options to use with the default boot option, but not with the
## alternatives
## e.g. defoptions=vga=791 resume=/dev/hda5
# defoptions=quiet splash

results in

title           Xen 3.2 / Ubuntu 7.10, kernel 2.6.18.8-xen
root            (hd0,0)
kernel          /xen-3.2.gz vtd=1
module          /vmlinuz-2.6.18.8-xen
root=UUID=2c9954c1-d8f5-477a-a808-39f768b10ac3 ro console=tty0 swiotlb=128
max_loop=64
module          /initrd.img-2.6.18.8-xen
quiet

after running update-grub.

Well, assuming you''ve appropriate xen, vmlinuz, initrd and System.map
files in place ;)


HTH

Cheers,

Stephan




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Rudi Ahlers wrote:
>> You can change it by modifying the file /etc/sysconfig/kernel:
>>
>> # UPDATEDEFAULT specifies if new-kernel-pkg should make
>> # new kernels the default
>> UPDATEDEFAULT=yes
>>
>> # DEFAULTKERNEL specifies the default kernel package type
>> DEFAULTKERNEL=kernel-xen
>>
>> Make sure that your default kernel is "kernel-xen" and not
"kernel"
>> obviously.
>>
>> Regards,
>>
>> Bas.
>>
> Cool, thank you. Does the kernel version affect this, or get affected by 
> this at all?
> 
When a new kernel is installed, the post install script touched the file 
to change old "kernel-smp" references to "kernel". Script
should only
update after a check on kernel-smp IMHO.

But if you have both xen-enabled and non-xen kernels installed, they 
both get updated when a new kernel of either type is available.

The /etc/sysconfig/kernel file only determines whether the newly 
installed kernel becomes the default. I personally don''t see why there 
are two switched needed for that. Guess it is for historical reasons.

Regards,

Bas.

(please keep the list posted)



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Bas Mevissen wrote:
> Rudi Ahlers wrote:
>
>>> You can change it by modifying the file /etc/sysconfig/kernel:
>>>
>>> # UPDATEDEFAULT specifies if new-kernel-pkg should make
>>> # new kernels the default
>>> UPDATEDEFAULT=yes
>>>
>>> # DEFAULTKERNEL specifies the default kernel package type
>>> DEFAULTKERNEL=kernel-xen
>>>
>>> Make sure that your default kernel is "kernel-xen" and
not "kernel"
>>> obviously.
>>>
>>> Regards,
>>>
>>> Bas.
>>>
>> Cool, thank you. Does the kernel version affect this, or get affected 
>> by this at all?
>>
> When a new kernel is installed, the post install script touched the 
> file to change old "kernel-smp" references to "kernel".
Script should
> only update after a check on kernel-smp IMHO.
>
> But if you have both xen-enabled and non-xen kernels installed, they 
> both get updated when a new kernel of either type is available.
>
> The /etc/sysconfig/kernel file only determines whether the newly 
> installed kernel becomes the default. I personally don''t see why
there
> are two switched needed for that. Guess it is for historical reasons.
>
> Regards,
>
> Bas.
>
> (please keep the list posted)
>
>
>
ok, now I have a tricky question. Do I need to have both kernels 
installed? What if I remove the standard kernel, and only keep the xen 
kernel?

-- 

Kind Regards
Rudi Ahlers
CEO, SoftDux

Web:   http://www.SoftDux.com
Check out my technical blog, http://blog.softdux.com for Linux or other
technical stuff, or visit http://www.WebHostingTalk.co.za for Web Hosting stuff


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Rudi Ahlers wrote:
> Bas Mevissen wrote:
> ok, now I have a tricky question. Do I need to have both kernels 
> installed? What if I remove the standard kernel, and only keep the xen 
> kernel?
> 
Well, I kept the non-xen kernel around in case the automatically 
starting xen domU''s would crash the system. In that case, I can easily 
start the box with a non-xen kernel to resolve the problem.

But technically, I see no reason to keep them around.

Bas.

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Bas Mevissen wrote:
> Rudi Ahlers wrote:
>> Bas Mevissen wrote:
>
>> ok, now I have a tricky question. Do I need to have both kernels 
>> installed? What if I remove the standard kernel, and only keep the 
>> xen kernel?
>>
>
> Well, I kept the non-xen kernel around in case the automatically 
> starting xen domU''s would crash the system. In that case, I can
easily
> start the box with a non-xen kernel to resolve the problem.
>
> But technically, I see no reason to keep them around.
>
> Bas.
>
That''s what I''m fearing as well. What if the Xen kernel
crashes. In this
case I''d still need to drive out to the DC - which I don''t
quite want. I
guess I''ll just exclude the normal kernel from the updates as you 
suggest. P.S. Do you know if one could set a backup / fail-safe kernel 
in grub, which can automatically boot if thew default kernel gives problems?

-- 

Kind Regards
Rudi Ahlers
CEO, SoftDux

Web:   http://www.SoftDux.com
Check out my technical blog, http://blog.softdux.com for Linux or other
technical stuff, or visit http://www.WebHostingTalk.co.za for Web Hosting stuff


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Rudi Ahlers wrote:
> That''s what I''m fearing as well. What if the Xen kernel
crashes. In this
> case I''d still need to drive out to the DC - which I
don''t quite want.
I''ve installed an IPMI2 module in my server. Costs about Eur 50 and 
gives you remote monitors (temperature, fans) and serial port-like 
console. Saved me a few trips to the DC already.
> I guess I''ll just exclude the normal kernel from the updates as
you
> suggest. P.S. Do you know if one could set a backup / fail-safe kernel 
> in grub, which can automatically boot if thew default kernel gives 
> problems?
> 
Just set UPDATEDEFAULT to "yes" if you want the latest Xen kernel by 
default and to "no" otherwise. Set DEFAULTKERNEL to
"kernel-xen" and
simply leave that other kernel around.

Grub is no watchdog. So it cannot detect boot failure. If you want 
something like that, arrange a budget and contact me :-)

Bas.

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Bas Mevissen wrote:
> Rudi Ahlers wrote:
>
>> That''s what I''m fearing as well. What if the Xen
kernel crashes. In
>> this case I''d still need to drive out to the DC - which I
don''t quite
>> want. 
>
> I''ve installed an IPMI2 module in my server. Costs about Eur 50
and
> gives you remote monitors (temperature, fans) and serial port-like 
> console. Saved me a few trips to the DC already.
>
>> I guess I''ll just exclude the normal kernel from the updates
as you
>> suggest. P.S. Do you know if one could set a backup / fail-safe 
>> kernel in grub, which can automatically boot if thew default kernel 
>> gives problems?
>>
>
> Just set UPDATEDEFAULT to "yes" if you want the latest Xen kernel
by
> default and to "no" otherwise. Set DEFAULTKERNEL to
"kernel-xen" and
> simply leave that other kernel around.
>
> Grub is no watchdog. So it cannot detect boot failure. If you want 
> something like that, arrange a budget and contact me :-)
>
> Bas.
>
Hi Bas

Unfortunately things work a lot different in our country. I''d like to 
take up your offer on the watchdog, but I know I won''t be able to
afford
it, especially if you say I need to budget for it. Our currency is about 
8times the USD, so for USD100 I need to pay USD800 - which is a lot. And 
then we still need to pay for import duties, etc. Quite often, the 
USD100 components end up costing us about USD300''s value.

P.S. What is IPMI2? Will it work on standard Pentium IV grade 
motherboards, or only on Xeon server grade motherboards? And does it get 
installed on the inside of the server, or how does it work?

-- 

Kind Regards
Rudi Ahlers
CEO, SoftDux

Web:   http://www.SoftDux.com
Check out my technical blog, http://blog.softdux.com for Linux or other
technical stuff, or visit http://www.WebHostingTalk.co.za for Web Hosting stuff


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Rudi Ahlers wrote:
> Unfortunately things work a lot different in our country. I''d like
to
> take up your offer on the watchdog, but I know I won''t be able to
afford
> it, especially if you say I need to budget for it. 
Maybe there is some soft watchdog solution already written. You will 
need some kind of remote power switch to make it complete.
> P.S. What is IPMI2? Will it work on standard Pentium IV grade 
> motherboards, or only on Xeon server grade motherboards? And does it get 
> installed on the inside of the server, or how does it work?
> 
http://en.wikipedia.org/wiki/Intelligent_Platform_Management_Interface
Remote console -> serial over LAN.
Depends on motherboard support, not on the CPU type. I use a rather 
cheap SuperMicro server that has IPMI with serial (console) over LAN as 
an option.

IPMI combines really good with Xen servers if you take a moment to 
configure it properly. Xen takes over the serial port, so you need to 
follow other instructions than for non-Xen systems. They are only 
slightly different luckily.

Regards,

Bas.

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> > I''ve installed an IPMI2 module in my server. Costs about Eur
50 and
> > gives you remote monitors (temperature, fans) and serial port-like
> > console. Saved me a few trips to the DC already.
> >
> >> I guess I''ll just exclude the normal kernel from the
updates as you
> >> suggest. P.S. Do you know if one could set a backup / fail-safe
> >> kernel in grub, which can automatically boot if thew default
kernel
> >> gives problems?
> >
> > Just set UPDATEDEFAULT to "yes" if you want the latest Xen
kernel by
> > default and to "no" otherwise. Set DEFAULTKERNEL to
"kernel-xen" and
> > simply leave that other kernel around.
> >
> > Grub is no watchdog. So it cannot detect boot failure. If you want
> > something like that, arrange a budget and contact me :-)
> >
> > Bas.
>
> Hi Bas
>
> Unfortunately things work a lot different in our country. I''d like
to
> take up your offer on the watchdog, but I know I won''t be able to
afford
> it, especially if you say I need to budget for it. Our currency is about
> 8times the USD, so for USD100 I need to pay USD800 - which is a lot. And
> then we still need to pay for import duties, etc. Quite often, the
> USD100 components end up costing us about USD300''s value.
>
> P.S. What is IPMI2? Will it work on standard Pentium IV grade
> motherboards, or only on Xeon server grade motherboards? And does it get
> installed on the inside of the server, or how does it work?
Grub can do some useful high availability things; you can set a fallback 
kernel in case the default fails to load at all.

You can also set up grub so that it will only choose a particular item *if* on 
the previous boot the system managed to get fully booted and ran a 
user-specified script to modify grub''s next boot settings.

This way you can ensure that if a system reboots before your grub update 
script gets run then the next boot will choose a safe fallback boot option.  
I set this up on one of my systems once.

Cheers,
Mark

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