filefrag--what's going on here?

Question

So I just found the command filefrag. I thought I'd see if any of my files are fragmented. I have quite a few large files, [500MB/+] and every single one I've tried has 5 to 42 extents found.

What's going on? I have EXT4, and I have 185G of 515G free [63% used].

I thought EXT3 and EXT4 didn't fragment files [unless absolutely necessary]. I did restore all my files actually, from a NTFS partition [My ext. hdd is NTFS. I've thought about formatting it to EXT, but it's 0.5TB and would take a long time to recopy my files] after I switched to a new laptop a week ago.

So how do I defragment these files, and why are they fragmented?

Answer 1

On the subject of extents:

Extents replace the traditional block mapping scheme used by ext2/3 filesystems. An extent is a range of contiguous physical blocks, improving large file performance and reducing fragmentation. A single extent in ext4 can map up to 128 MiB of contiguous space with a 4 KiB block size.

(from wikipedia)

128 Mb per extent gives about 4 extents per 500Mb file for absolutely the best possible scenario. Are your "more fragmented" files in 5Gb size range? Also, this doesn't necessary mean you have that many non-continuous regions... extents may follow each other on disk.

I believe fragmentation is an issue when you have thousands of non-continuous regions each of which is a few kilobytes in size. If your extents are many tens or hundreds of megabytes in size any effects on performance are negligible. Relax.