The "Tota/best extents" calculation for done in the e4defrag program doesn't seems to not consider the limit of 32768 file system blocks (see /usr/src/linux/fs/ext4/ext4_extents.h comment EXT_INIT_MAX_LEN) in an extent when computing the "best" value. It seems to presume 2G is the best extent size, but on a file system with a 4kb block size the largest available extent is 128M not 2G.
(The best extent size calculation in e4defrag seems to be based on block group size but the underlying implementaiton in the kernel is limited to a __le16 blocks and uses up one whole bit of that to flag whether the extent is initialized, which actually produces a one-block purturbation in the actual limit based on how the extent was allocated (e.g. whether falloc used FS_KEEP_SIZE or not), go go gadget purturbed math. 8-)).
-rw-r--r-- 1 rwhite rwhite 5.0G Jun 13 16:28 CentOS.qcow2
yeilds output: (where "/3" is wrong).
Total/best extents 152/3
The e4defrag program should
ASIDE: I -thought- I was supposed to be able to make 2G extents in ext4 but on intel platforms with a 4kb memory page, the file system block size is capped at 4kb as well which caps the extent length at 128Mb. The 2G limit is apparently for processors with larger memory pages and therefore file system block sizes. I was bummed to learn this. I suspect that this is a case of false advertising by the ext4 guys not tracing all their dependencies and limits. /sigh. 8-)