Sequential read / write speed
While random I/O speed is crucial for system responsiveness and overall performance, sequential speed is important also: we are often dealing with big files, on which read / write operations are done with bigger block sizes.
Will BTRFS redeem itself?
No, not really.
This time, using default mount options leads to very slow read and write performances. If preallocation did nothing to alleviate the situation, disable CoW and BTRFS can enjoy a tremendous performance improvement.
So, it seems that the single greatest problem with virtual images and BTRFS is the CoW behavior. But why? We are going to answer this question, but let first see the fragmentation results.