Linux – Detailed sparse file information on Linux

Ext4 can use 1kB, 2kB or 4kB as the block size; as far as I know the default on Ubuntu is 4kB. Note that here, a block is the size of a file chunk, which is constant for a given filesystem. The file you describe has two blocks that are not zeroes: the one containing hello (surrounded by a bunch of zeroes — 3616 before and 474 after), and the one containing here (preceded by a bunch of zeroes, and containing only 3148 bytes, after which the end of the file is reached). The total is two blocks of 4kB.

In the ls output, blocks are an arbitrary unit chosen by the ls command and defaulting to 1kB. There are 2 blocks of 4kB each allocated to contain file data, therefore the allocated size for the file is 8kB.

Your confusion may be due to two things. First, the figure of 2048 bytes for a block is possible, but it's not the default value under Ubuntu (or most modern distributions), and it's apparently not the value on your system. You can check the block size by running tune2fs -l /dev/sdz42 (use the actual path to your filesystem device).

Second, sparse files consist of not storing blocks that are entirely made of zeroes. If a block (which is of necessity aligned on a block size boundary, at least for most filesystems including ext4) contains zeroes and other things, then the full block is stored on the disk. Thus, in that 40012-byte file (how did you get to 40013, by the way), there are 4 all-zero non-stored blocks, then one stored block containing hello surrounded by zeroes, then 4 more all-zero non-stored blocks, and a final partial block containing zeroes and there.

Note that your utility can be written in terms of standard shell commands:

n=20000
while IFS= read -r line; do
  dd bs=1 seek=$n </dev/null
  echo "$line"
done >testfile

The best I could come up with so far is (ksh93, using filefrag from e2fsprogs 1.42.9 (some older versions have a different API), on extent based file systems on Linux):

#! /bin/ksh93
export LC_ALL=C
for file do
filefrag -vb1 -- "$file" |
  while IFS=": ." read -A a; do
    [[ $a = +([0-9]) ]] && [[ ${a[@]} != *unwritten* ]] &&
      command /opt/ast/bin/head -s "${a[1]}" -c "${a[7]}" -- "$file"
  done
done

filefrag reports the extents of the file using the FIEMAP ioctl for the filesystems that support it.

The *unwritten* part covers for the (non-sparse, but still full of zeros I'm not interested in) files that have been fallocated but not written to.

Recent versions of bsdtar or star can use some of those APIs to generate a tar file that identifies the sparse sections as such. That would make for a more portable solution, but then one would have to parse the generated tar file to get the non-sparse sections.