Re: fiemap is broken for sparse file in ext4?

From: Tao Ma
Date: Fri Jun 11 2010 - 01:06:57 EST

Hi Eric,
Thanks for the quick response.

On 06/11/2010 12:08 PM, Eric Sandeen wrote:
Tao Ma wrote:
Hi Ted and other ext4 gurus,
I found fiemap may be broken for sparse files in ext4. Here is a
simple example.

dd if=/dev/zero of=testfile1 bs=1M count=1
using fiemap shows that it has a delalloc extent.
Logical: 0 Ext length: 1048576 Physical: 0 flags: 7


while if we create a sparse file, fiemap will not show the delalloc extent.
dd if=/dev/zero of=testfile1 bs=1M count=1 seek=1
using fiemap shows that it has no extent for the file. while we should
have some output like:
Logical: 1048576 Ext length: 1048576 Physical: 0 flags: 7

So we have different output with sparse and non-sparse file. Is it a bug
for ext4?

What are you using to call fiemap? Here it seems to be working:
I just wrote a simple test program by calling ioctl. It is attached. btw, you need to call it immediately after dd so that we have a chance that ext4 don't have time to allocate extents. ;)

# dd if=/dev/zero of=testfile1 bs=1M count=1 seek=1;

# filefrag -v testfile1
Filesystem type is: ef53
Filesystem cylinder groups is approximately 119
File size of testfile1 is 2097152 (512 blocks, blocksize 4096)
ext logical physical expected length flags
0 256 151946 1 merged
1 257 151951 151946 2 merged
2 259 152434 151952 253 merged,eof
testfile1: 4 extents found
I guess maybe filefrag use the diffrent ioctl flag, maybe FIEMAP_FLAG_SYNC to let ext4 sync first.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <stdint.h>
#include <inttypes.h>
#include <string.h>

#include "fiemap.h"

#define FS_IOC_FIEMAP _IOWR('f', 11, struct fiemap)

static unsigned int num_extents = 1024;
static unsigned int estimate_extents = 1;

static unsigned long long map_start = 0ULL;
static unsigned long long map_len = FIEMAP_MAX_OFFSET;
static char *fname;
static unsigned int blocksize = 4096;

static void usage(void)
printf("Usage: fiemap filename\n");

static void print_extent(struct fiemap_extent *extent)
__u64 val;

printf("Logical: ###[%8"PRIu64"]\t", extent->fe_logical);
printf("Ext length: ###[%8"PRIu64"]\t", extent->fe_length);
printf("Physical: ###[%8"PRIu64"]\t", extent->fe_physical);
printf("flags: %u\t", extent->fe_flags);

if (extent->fe_flags & FIEMAP_EXTENT_SHARED)
printf("Yeah, found an shared extent\n");
if (extent->fe_flags & FIEMAP_EXTENT_UNWRITTEN)
printf("Yeah, you are hole?\n");


static void show_results(struct fiemap *fiemap)
int i;

printf("Extents returned: %u\n", fiemap->fm_mapped_extents);
if (fiemap->fm_extent_count == 0 || fiemap->fm_mapped_extents == 0)

for (i = 0; i < fiemap->fm_mapped_extents; i++)


static int
figure_extents(int fd, unsigned int *num)
int ret;
struct fiemap fiemap = { 0, };

fiemap.fm_start = map_start;
fiemap.fm_length = map_len;
fiemap.fm_flags = 0;

ret = ioctl(fd, FS_IOC_FIEMAP, &fiemap);
if (ret == -1) {
fprintf(stderr, "fiemap get count error %d: \"%s\"\n", errno,
return -1;

*num = fiemap.fm_mapped_extents;

return 0;

main(int argc, char **argv)
int ret, fd;
struct fiemap *fiemap;

fd = open(argv[1], O_RDONLY);
if (fd == -1) {
fprintf(stderr, "open error %d: \"%s\"\n", errno,
return -1;

if (figure_extents(fd, &num_extents))
return -1;

printf("Extents in file \"%s\": %u\n", argv[1], num_extents);

fiemap = malloc(sizeof(fiemap) +
num_extents * sizeof(struct fiemap_extent));
if (fiemap == NULL) {
fprintf(stderr, "malloc error %d: \"%s\"\n", errno,
return -1;

fiemap->fm_start = map_start;
fiemap->fm_length = map_len;
fiemap->fm_extent_count = num_extents;

ret = ioctl(fd, FS_IOC_FIEMAP, fiemap);
if (ret == -1) {
if (errno == EBADR) {
fprintf(stderr, "Kernel does not support the flags: 0x%x\n",
return -1;
fprintf(stderr, "fiemap error %d: \"%s\"\n", errno,
return -1;



return 0;