Comment 44 for bug 321528

Oh, and concerning ext4 fragmentation, I find it strange that so much fragmentation as cited above... The most fragmented ext4 FS I have is about 8%, and strangely, another one which is almost full and its content moderately often changed, stays low at about 2% even though I've expected much more.

I believe such a low degree of fragmentation I have is due to Extents, and I have specifically enabled them for my ext4 filesystems. Perhaps in those cases with higher ext4 fragmentation extents are turned off? I'm not sure are they enabled by default, but I believe they are disabled if the FS is upgraded from ext3, so I'd suggest anyone to manually enable it (in fstab, perhaps also with tune2fs); I'd recommend rebooting to single mode immediately after that, and then manually fsck all filesystems for which extents were turned on. And remember, backup often ;)

Also, when extents are turned on, perhaps running the Defrag script or Shake would help lower the fragmentation...