[vivekl]

I think the problem also depends on what kind of virtual disk you end up using. Let me ellaborate:

I dont think the problem is purely buffered vs. unbuffered IO. The guest operating system will have performed some block coalescing anyway so the block requests will often NOT be 4K chunks but should have slightly larger granularity. However, if you use a COW based virtual disk layout like QCOW2 which I guess is standard in KVM, you may see additional scattered IO.

I think it is weird to be using COW virtual disk layouts on a file system that natively supports COW as is the case with BTRFS. I would be curious to see what the performance of raw sparse files on BTRFS is vs. qcow2 etc.