Re: Set nodatacow per file?

[Roman Mamedov <rm@romanrm.ru>]

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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.

-- 
With respect,
Roman

~~~~~~~~~~~~~~~~~~~~~~~~~~~
"Stallman had a printer,
with code he could not see.
So he began to tinker,
and set the software free."

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#include <fcntl.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <string.h>

#include <linux/types.h>
#include <stdio.h>
#include <errno.h>

#ifndef FS_IOC_SETFLAGS
#define FS_IOC_SETFLAGS                 _IOW('f', 2, long)
#warning defining SETFLAGS locally
#endif

#ifndef FS_IOC_GETFLAGS
#define FS_IOC_GETFLAGS                 _IOR('f', 1, long)
#warning defining GETFLAGS locally
#endif

#ifndef FS_NOCOW_FL
#define FS_NOCOW_FL                     0x00800000 /* Do not cow file */
#endif

int main(int argc, char **argv)
{
        int fd;
        int r;
  long flags;

  if (argc < 2) {
    printf("usage: %s file\n", argv[0]);
    exit(1);
  }

  fd = open(argv[1], O_RDONLY);
  if (fd == -1) {
    perror("open()");
    return 1;
  }
  printf("GETFLAGS ioctl 0x%x\n", FS_IOC_GETFLAGS);
  printf("SETFLAGS ioctl 0x%x\n", FS_IOC_SETFLAGS);

  r = ioctl(fd, FS_IOC_GETFLAGS, &flags);
  if (r == -1) {
    perror("ioctl(GETFLAGS)");
    return 1;
  } else {
    printf("file flags before: 0x%lx\n", flags);
  }

  if(strcmp(argv[0], "cow")==0) {
    printf("Remove NOCOW flag for %s\n", argv[1]);
    flags ^= FS_NOCOW_FL;
  } else {
    printf("Set NOCOW flag for %s\n", argv[1]);    
    flags |= FS_NOCOW_FL;
  }
  r = ioctl(fd, FS_IOC_SETFLAGS, &flags);
  printf("ioctl returned: %d\n", r);
  if (r == -1) {
    perror("ioctl()");
    return 1;
  }

  r = ioctl(fd, FS_IOC_GETFLAGS, &flags);
  if (r == -1) {
    perror("ioctl(GETFLAGS)");
    return 1;
  } else {
    printf("file flags after:  0x%lx\n", flags);
  }  

  return 0;
}

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