[CentOS-virt] xen device mapping/translation

Luis Fernando Alen

luis.alen at izap.com.br
Wed Jan 16 15:04:49 UTC 2013


Andy,

Actually I'm not trying to build a whole new kernel. I'm just trying to
apply the patched module into my actual kernel.

Does this patch really requires a kernel rebuild, or you mean building a
new one will save me from the trouble of applying the module into the
running kernel?

Luis Alen
www.izap.com.br
Ligue com tarifa local de todo o Brasil 4020.3000



On Wed, Jan 16, 2013 at 10:57 AM, Nico Kadel-Garcia <nkadel at gmail.com>wrote:

>
>
> On Tue, Jan 15, 2013 at 8:39 PM, Luis Fernando Alen <luis.alen at izap.com.br
> > wrote:
>
>> Thank you, Andy.
>>
>> I tried to apply the patch you guys mentioned by compiling the module
>> following instructions at
>> http://wiki.centos.org/HowTos/BuildingKernelModules#head-d2e4c05886f94c701e4ae74387d41d8c40c25d01,
>> but it didn't work.
>>
>> I've been struggling with it for the last 8 hours and no luck so far.
>>
>> I really don't know what's wrong. I'm not a linux kernel developer and
>> I'm most likely failing because of something stupid.
>>
>> I know this must not the right place to ask for help on such matters, but
>> if you guys could shed some light here, I'd really appreciate that.
>>
>> Well, if you're up to it, here's the situation:
>>
>> Looks like the module compilation worked (no errors or warnings occurred
>> when I followed the instructions at the centos wiki), but I'm unable to
>> load the new module to my running kernel.
>>
>
> If you're building a new kernel, you should really give it a new name and
> fully install it as a distinct kernel. The safest way to do this is to work
> from the SRPM, put the patch in *there* and  update the "Release:"
> number in the kenrel.spec file This will avoid precisely the issues you
> described.
>
> Do you need a walkt hrough on rebuilding a package from SRPM's?
>
>
>> I even copied the compiled and patched module to
>> /lib/modules/2.6.32-279.19.1.el6.x86_64/kernel/drivers/block/ (overwrote
>> the original) and /lib/modules/2.6.32-279.19.1.el6.x86_64/extra and
>> rebooted the instance...
>>
>> Also, dmesg does not complain about a thing...
>>
>> *# modinfo
>> /lib/modules/2.6.32-279.19.1.el6.x86_64/kernel/drivers/block/xen-blkfront.ko
>> *
>> *filename:
>> /lib/modules/2.6.32-279.19.1.el6.x86_64/kernel/drivers/block/xen-blkfront.ko
>> *
>> *alias:          xenblk*
>> *alias:          xen:vbd*
>> *alias:          block-major-202-**
>> *license:        GPL*
>> *description:    Xen virtual block device frontend*
>> *srcversion:     B00B4183E470515A96DA320*
>> *depends:        *
>> *vermagic:       2.6.32-279.19.1.el6.x86_64 SMP mod_unload modversions *
>> *parm:           sda_is_xvda:sdX in guest config translates to xvdX, not
>> xvd(X+4) (bool)*
>> *
>> *
>> *# uname -r*
>> *2.6.32-279.19.1.el6.x86_64*
>>
>> I also tried to remove the module and insert the patched one with insmod,
>> but modprobe and rmmod are unable to unload it. They say it's in use.
>>
>> *# lsmod |grep blkfront*
>> *xen_blkfront           15495  1 *
>>
>> I don't know what this "1" stands for, but if I were to guess, I'd say
>> it's something unremovable...
>>
>> Please let me know if you need any other information.
>>
>> Thanks,
>>
>>
>>
>>
>>
>> Luis Alen
>> www.izap.com.br
>> Ligue com tarifa local de todo o Brasil 4020.3000
>>
>>
>>
>> On Tue, Jan 15, 2013 at 4:13 PM, Andy Grimm <agrimm at gmail.com> wrote:
>>
>>> See https://bugzilla.redhat.com/show_bug.cgi?id=729586
>>>
>>> On Tue, Jan 15, 2013 at 1:10 PM, Luis Fernando Alen <
>>> luis.alen at izap.com.br> wrote:
>>>
>>>> Hello, list.
>>>>
>>>> Yesterday I was pleased to see that Centos has released official images
>>>> at the aws marketplace. Nice job.
>>>>
>>>> Today I started playing with the Centos 6.3 image (
>>>> https://aws.amazon.com/marketplace/pp/B00A6L6F9I, on which I plan to
>>>> deploy a gluster cluster in production soon) and noticed a weird thing.
>>>>
>>>> EBS Volumes attached to sd<X> are translated to xvd<Y> at the OS level.
>>>> However, after a few research and IRC chat, I figured out that it's not
>>>> weird, it's actually a normal and expected behavior (thanks for your help,
>>>> z00dax).
>>>>
>>>> sdX is actually mapped to xvdX+4. There is a consistent offset of 4.
>>>> Suppose you attach an ebs volume to /dev/sdf. It'll be translated to xvdj
>>>> at the OS level. sdg to xvdk, sdh to xvdl and so on.
>>>>
>>>> Allright. After having figured the mystery out, it became easy to work
>>>> on automations that deal with ebs volumes and file systems, such as volumes
>>>> created, attached and mounted on the fly, snapshots that freeze file
>>>> systems and so on...
>>>>
>>>> However, I really do think to myself: Wouldn't it be cleaner if the
>>>> image use simple translation (sdX to xvdX)? If I'm not wrong, Rightscale
>>>> uses this on their Centos images and it's much simpler. There's no extra
>>>> work needed to deal with that 4 offset when you want to automate things.
>>>>
>>>> Is there a reasonable reason for the 4 offset which makes it
>>>> unchangeable?
>>>>
>>>> It's just a thought. I think it's worth considering it..
>>>>
>>>> Luis Alen
>>>> www.izap.com.br
>>>> Ligue com tarifa local de todo o Brasil 4020.3000
>>>>
>>>>
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>>>> CentOS-virt mailing list
>>>> CentOS-virt at centos.org
>>>> http://lists.centos.org/mailman/listinfo/centos-virt
>>>>
>>>>
>>>
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>>
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>
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