Fiemap retrieval on sparse file with multiple extents

The fiemap is an efficient way for userspace to get file extent mappings. Instead of block-by-block mapping, fiemap returns a list of extents.

• Generate sparse file with multiple extents.

  make
  ./generator
  

• Install fiemap tool.

  sudo apt install gcc make -y
  wget https://github.com/joshimoo/fiemap/archive/refs/tags/v0.2.0.tar.gz
  tar zxvf v0.2.0.tar.gz
  cd fiemap-0.2.0
  make
  sudo make install
  

• Use ls -lsah to see the sparse file actual used space as well as max space. Example below 1GB allocated, 514MB used.

  514M -rw-r--r-- 1 jenting users 1.0G Mar 17 08:36 /tmp/  ssync-src-fiemap-1gb-file
  

• Then run fiemap.

  fiemap /tmp/ssync-src-fiemap-1gb-file
  

  If the initial 1024 retrieval takes around 3 minutes you can cancel it via ctrl+c this will take another 3 minutes till the process returns from the kernel. This is a slow EXT4 extent retrieval -> Bad Kernel. If the initial 1024 retrievals takes 0 seconds than you are good and can let it run to completion. -> Good Kernel.

  Example of a full run on a newer 5.8 kernel

  retrieved 1024 extents in 0 seconds
  
  fiemap done retrieved 131072 extents in 39 seconds
  File /tmp/ssync-src-fiemap-1gb-file has 131072 extents: