I have a Ubuntu 14.04 application appliance that makes heavy use of BTRFS snapshots. The application is meant to be hypervisor agnostic and the snapshots need to be stored with the virtual machine image in case we need to troubleshoot an issue. Using the hypervisors built in snapshotting methods instead of BTRFS snapshotting would be more work than it's worth since it would require API access to the hypervisor from the VM which we don't want for security reasons. I can also remotely access the BTRFS snapshot filesystem subvolumes directly from the appliances command line via ssh without having to power down the machine or access a hypervisor API.
In the past I've only had to deal with deploying this application to vmware based hypervisors. I always used thin provisioning on my vmware disks and never noticed any performance issues. I use thin provisioning because I do a large amount of testing on this appliance and I tend to deploy many appliances at a time to run different tests in parallel.
I'm very careful about not over committing the storage by using scripts that ensure the disk growth doesn't run out of control. The I/O dip that happens when the thin provisioned disk needs to grow isn't all that noticeable either.
Now I need to support KVM and would very much like to keep Thin/sparse provisioning my disks, however I've read a few things that state mixing a CoW filesystem with another CoW filesystem is a bad idea due to over-redundant writes and disk fragmentation among other things. The common example given was running a VM with a qcow2 formatted disk stored on a BTRFS formatted volume. My situation would be the opposite. I want to have a BTRFS formatted filesystem in a VM running from a qcow2 image. I haven't found much on the particulars of the performance impact of BTRFS snapshots on top of a qcow2 image.
Question: Are there any other sparse file formats that grow with the disk size that KVM likes to use?
I've explored with using sparse raw files, but I can't seem to get them to stay sparse through a cp, download untar/gunzip, ect. It seems that if you want to use a sparse raw file, you can't move it around at all which would make distribution a pain in the ass.