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Re: [Libguestfs] New tool proposal



On 05/23/2012 05:09 PM, Richard W.M. Jones wrote:

> On Wed, May 23, 2012 at 04:45:22PM +0800, Wanlong Gao wrote:
>> Kame gave me a very good detail about the use case of virt-diff example, now
>> quoted them for you,
>>
>>> Anyway, in above scheme, we cannot compare the files of the same size.
> [...]
> 
> virt-ls can find the differences if you use the --checksum option,
> because this computes a checksum over the whole file content.  If its
> just the file date which has changed, it's cheaper to use the --times
> option.
> 
> http://libguestfs.org/virt-ls.1.html#checksum
> http://libguestfs.org/virt-ls.1.html#recursive_long_listing
> 
>>> In another point of view....
>>>
>>> In your example in mailing list, you compares 2 files.
>>>
>>> But, now, I _think_ what users really want is a command like following.
>>>
>>> Following command is comparing guest's file of Guest01...Guest04 with Guest00.
>>>
>>> # virt-diff -seed Guest00 -name Guest01 -name Guest02 -name Guest03 -name Guest04 /etc/hogehoge
>>> Guest03:/etc/hogehoge is differ.
>>> diff result...
>>>
>>> In this case, only Guest03 has a different contents of the file.
>>>
>>> For system-admin, this means...
>>> - Guest00,01,02,04 are correct and 03 is wrong
>>> or
>>> - Guest03 is correct and Guest00,01,02,04 are wrong.
>>> or
>>> - Guest03 has different settings intentionally and he confirmed that.
>>>
>>> This command will be very good for system-admin.
>>
>> Then, any thoughts about this tool ?
> 
> It sounds useful, but I think you could still implement this on top of
> virt-ls.
> 
>> and more thoughts about implementing this ?
>> We should implement a "diff" wrapper in the guestfsd, and use this API when
>> implementing this tool or any other methods?
> 
> The problem with adding 'diff' as an operation to the daemon is that
> you would need to attach the disks from multiple guests to a single
> appliance.  This is in general problematic, although we do it in some
> limited cases:
> 
> http://libguestfs.org/guestfs-performance.1.html#reducing_the_number_of_times_the_appliance_is_launched
> 
> If you wanted to write a separate virt-diff tool, it's probably a good
> idea to start with the source of virt-ls and work from there.  That
> program already knows how to iterate over the filesystem (of a single
> guest), and you could build diff functionality on top of that:
> 
> https://github.com/libguestfs/libguestfs/blob/0e05e9f6ce696cf2a5172eda3fe054367164106b/cat/virt-ls.c#L509


OK, thank you very much for your opinion, I'll look into it and try to do
as what you suggested.

Regards,
Wanlong Gao

> 
> Rich.
> 



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