how diff between hardlink trees works?

[Vaibhav Jain]

Thanks a lot!
Now it got clear to me.


On Fri, Sep 9, 2011 at 7:47 PM, Kai Meyer <kai at gnukai.com> wrote:

> **
> On 09/09/2011 12:39 PM, Vaibhav Jain wrote:
>
>
>
> On Fri, Sep 9, 2011 at 11:12 AM, Kai Meyer <kai at gnukai.com> wrote:
>
>>   On 09/09/2011 09:05 AM, Vaibhav Jain wrote:
>>
>>  Hi,
>>
>> I am not able to understand how diff between two trees of which one is
>> just contains hardlinks to another's files (cp -al )ing
>> works.I am asking this question here because I need to build a custom
>> kernel for which I need to generate patch. So the
>> documentation suggests to create a hardlink copy of the kernel source tree
>> using cp -al and then make changes to
>> one of the trees and run a diff.I am wondering that if files are hardlinks
>> then changes to one copy will affect another in which case
>> diff should give no output.
>> Also, the patch I created looks a little odd as it contains complete
>> modified files instead of just the differences.
>> Please help!
>>
>> Thanks
>> Vaibhav Jain
>>
>>
>> _______________________________________________
>> Kernelnewbies mailing listKernelnewbies at kernelnewbies.orghttp://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>>
>>  Make the hard link copy like normal. Then delete the directory that you
>> are making changes to (in the hard link directory), then copy the files over
>> with out hard links. That way "most" of the kernel tree is hard linked, and
>> just the portion you want to work on is a copy. That way the diff will work.
>>
>> Otherwise, skip the hard link part all together, and just make a full
>> copy. Uses lots of disk space and takes longer to diff.
>>
>> -Kai Meyer
>>
>
>
> Hi Kai,
>
> Thanks for the reply. I need just one more favour.
> Could you please look at this document describing the procedure to build
> custom fedora kernel. It mentions the step to create hardlink to generate
> but doesn't
> talk about deleting anything ?I just need to confirm if the article is not
> accurate or if there is
> any error in my understanding.
> Whenever I follow it I get a patch that contains all of the content of the
> changed files rather than just the changes.
>
> Here is the relevant portion :
>  Copy the Source Tree and Generate a Patch
>
> This step is for applying a patch to the kernel source. If a patch is not
> needed, proceed to "Configure Kernel Options".
>
> Copy the source tree to preserve the original tree while making changes to
> the copy:
>
> cp -r ~/rpmbuild/BUILD/kernel-2.6.$ver.$fedver/linux-2.6.$ver.$arch ~/rpmbuild/BUILD/kernel-2.6.$ver$fedver.orig
> cp -al ~/rpmbuild/BUILD/kernel-2.6.$ver.$fedver.orig ~/rpmbuild/BUILD/kernel-2.6.$ver.$fedver.new
>
>
> * The second cp command hardlinks the .orig and .new trees to make diffrun faster. Most text editors know how to break the hardlink correctly to
> avoid problems.*
>
> Using vim on FC14, it treated the hard link as a hard link and thus the
> above technique failed. It was necessary to repeat the original copy used
> for the .orig directory for the .new directory. Note that this uses twice
> the space.
>
> Make changes directly to the code in the .new source tree, or copy in a
> modified file. This file might come from a developer who has requested a
> test, from the upstream kernel sources, or from a different distribution.
>
> After the .new source tree is modified, generate a patch. To generate the
> patch, run diff against the entire .new and .orig source trees with the
> following command:
>
> cd ~/rpmbuild/BUILD
> diff -uNrp kernel-2.6.$ver.$fedver.orig kernel-2.6.$ver.$fedver.new > ../SOURCES/linux-2.6-my-new-patch.patch
>
>
> Thanks
> Vaibhav
>
>
> _______________________________________________
> Kernelnewbies mailing listKernelnewbies at kernelnewbies.orghttp://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
>
> The article says this:
>
> "Using vim on FC14, it treated the hard link as a hard link and thus the
> above technique failed. It was necessary to repeat the original copy used
> for the .orig directory for the .new directory. Note that this uses twice
> the space."
> It means to say that some editors, like VIM, edit files in-place, and some
> files copy the original contents into some other buffer (memory or temporary
> file), and then effectively delete the file you're editing, and copy the
> modified file into place. The hard-link instructions are a "trick" to save
> time and space when you are modifying large code base, like the kernel. If
> your favorite editor is behaving like the observed behavor of VIM, then you
> will need to delete the hard link file, and put a regular copy of the file
> in place before making changes.
>
> -Kai Meyer
>
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