Large files, nodatacow and fragmentation

["G. Richard Bellamy" <rbellamy@pteradigm.com>]

I've been playing with btrfs as a backing store for my KVM images.

I've used 'chattr +C' on the directory where those images are stored.

You can see my recipe below [1]. I've read the gotchas found here [2]

I'm having continuing performance issues inside the Guest VM that is
created inside the btrfs subvolume, using a qcow2 format. I'm having a
hard time determining whether the issues are related to KVM or btrfs,
or if this is even a reasonable topic of discussion.

I've seen the comments on this list saying that if I want a COW
filesystem with sparse files, that I'd be better off with ZFS. I'd
like to use an in-tree COW filesystem, but if it's just not gonna
happen yet with btrfs, I guess that's just the way it is.

That being said, how would I determine what the root issue is?
Specifically, the qcow2 file in question seems to have increasing
fragmentation, even with the No_COW attr.

[1]
$ mkfs.btrfs -m raid10 -d raid10 /dev/sda /dev/sdb /dev/sdc /dev/sdd
$ mount /dev/sda /mnt
$ cd /mnt
$ btrfs create subvolume __data
$ btrfs create subvolume __data/libvirt
$ cd /
$ umount /mnt
$ mount /dev/sda /var/lib/libvirt
$ chattr +C /var/lib/libvirt/images
$ cp /run/media/rbellamy/433acf1d-a1a4-4596-a6a7-005e643b24e0/libvirt/images/atlas.qcow2
/var/lib/libvirt/images/
$ filefrag /var/lib/libvirt/images/atlas.qcow2
/var/lib/libvirt/images/atlas.qcow2: 0 extents found
[START UP THE VM - DO SOME THINGS]
$ filefrag /var/lib/libvirt/images/atlas.qcow2
/var/lib/libvirt/images/atlas.qcow2: 12236 extents found
[START UP THE VM - DO SOME THINGS]
$ filefrag /var/lib/libvirt/images/atlas.qcow2
/var/lib/libvirt/images/atlas.qcow2: 34988 extents found

[2]
https://btrfs.wiki.kernel.org/index.php/Gotchas