EXT4 is usually pretty good at keeping files fragmentation at minimum, but, sometimes, especially if dealing with really huge files, some fragmentation may actually occur.
Luckily EXT4 supports online defragmentation, command fsck displays, among other things, fragmentation percentage:
[root@fedora ~]$ fsck.ext4 -fvn /dev/sda1
e2fsck 1.42.12 (29-Aug-2014)
Warning! /dev/sda1 is mounted.
Warning: skipping journal recovery because doing a read-only filesystem check.
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
429 inodes used (1.31%, out of 32768)
**5 non-contiguous files (1.2%)**
1 non-contiguous directory (0.2%)
# of inodes with ind/dind/tind blocks: 0/0/0
Extent depth histogram: 420
45161 blocks used (34.46%, out of 131072)
0 bad blocks
1 large file
402 regular files
17 directories
0 character device files
0 block device files
0 fifos
0 links
1 symbolic link (1 fast symbolic link)
0 sockets
------------
420 files The command e4defrag, which is contained in e2fsprogs, can be used to perform online defragmentation of EXT4 volumes.
…