as requested. like this? 


---begin
#define _GNU_SOURCE             /* See feature_test_macros(7) */
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <linux/falloc.h>
#include <unistd.h>

#define BLOCKSIZE 4096

int main(int argc, char** argv) {
    ssize_t rsize;
    int fh, result;
    char buff[BLOCKSIZE], ebuff[BLOCKSIZE];
    off_t curpos, pstart, psize;
    int freed;

    if (argc < 2) {
	printf("%s [file...]\n", argv[0]);
	return 0;
    } 

    memset(ebuff, 0, BLOCKSIZE);	// prepare a block of 0's
    
    for (int i = 1; i < argc; i++) {
	char* file=argv[i];
	curpos = 0;
	pstart = 0;
	psize = 0;
	freed = 0;

	fh = open(file, O_RDWR, O_NOATIME);
	if (fh == -1) {
	    perror("open()");
	    return -1;
	}
    
	printf("sparseifying %s ", file);
	fflush(stdout);
	while ((rsize = read(fh, buff, BLOCKSIZE)) > 0) {
	    result = memcmp(buff, ebuff,rsize);
	    if (result == 0) { // block is empty
		if (pstart == 0) { // previous block as not empty?
		    pstart = curpos;
		    psize = rsize; // save for later punching
		} else {
		    psize += rsize; // previous block was empty too, add size
		}
	    
		freed += rsize;
	    } else if (pstart) { // block is not empty and we have a block that we still need to punch
		result = fallocate(fh, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, pstart, psize);
		if (result == -1) {
		    perror("fallocate()");
		    return -1;
		}
		pstart = 0;
		psize = 0;
	    }
	    curpos += rsize;
	}

	if (pstart) { // still a block to do ?
	    result = fallocate(fh, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, pstart, psize);
	    if (result == -1) {
		perror("fallocate()");
		return -1;
	    }
	}

	if (rsize == 0) {
	    printf("done. freed %i bytes\n", freed);
	    continue;
	}
	if (rsize == -1) {
	    perror("read()");
	    return -1;
	}
    }
    return -1;
}
---end


Remco Hosman


On Nov 18, 2012, at 10:19 PM, Hugo Mills [off-list ref] wrote:

On Sun, Nov 18, 2012 at 10:04:29PM +0100, Remco Hosman - Yerf IT wrote:

quoted

I wrote a little tool you can use to scan a file and punch holes, so make it sparse.

Confirmed to work on 3.7.0-rc6
Feel free to use it in any way you like, and of course improve it. 

http://pastebin.com/8SjEsBLD

  For archival purposes, it's probably better to put the whole thing
inline in the text of your mail. This also makes it far easier to make
comments on it, should anyone feel moved to do so.

  Hugo.

-- 
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        --- In theory, theory and practice are the same. In ---         
                     practice,  they're different.