Re: Set nodatacow per file?

[dima <dolenin@parallels.com>]

On 02/13/2012 05:09 PM, Roman Mamedov wrote:
> On Mon, 13 Feb 2012 16:40:03 +0900
> dima<dolenin@parallels.com>  wrote:
>
>> Hello Ralf-Peter,
>>
>> Actually it is possible. Check out David's response to my question from
>> some time ago:
>> http://permalink.gmane.org/gmane.comp.file-systems.btrfs/14227
>>
>> The nocow.c script he attached does just the thing you want. The script
>> is really working. I needed nocow for different purpose but it did not
>> occur to me to try it on VM image and see if the performance would
>> improve. Sounds like a great idea. If you get around to try it, pls.
>> post your impressions here.
>
> Thanks for the link, this is indeed interesting.
>
> I made a couple of small changes, i.e. I wanted a way to unset nocow and to
> check that changing flags really worked.
>
> gcc -o /usr/local/bin/nocow nocow.c
> ln -sf /usr/local/bin/nocow /usr/local/bin/cow
>
> Perhaps the support for setting this flag should be added to the 'btrfs'
> utility.
>

Hello Roman,
I don't seem to be able to 'unset' the NOCOW flag.  Looking at the code 
I would guess that it is supposed to alternate between 'cow' and 'nocow' 
states, but the condition
     printf("Remove NOCOW flag for %s\n", argv[1]);
never shows for me. What should I do to make it working?
Maybe it would be nice to have a switch to just check if nocow is set on 
file without actually changing the flag.

thanks
~dima