Ceph » bluestore

Indeed it hardly scales as currently it keeps every free extent using 16 bytes (offset + size). Hence 256M extents will use ~4GB. Thus current format's drawbacks are:
1) processing time/space usage are volatile and grow with disk fragmentation
2) 4GB is a huge volume and it's effectively a max size for bluefs file size.
3) Modern large disks (10-20TB) can easily achieve the limits

Apparently bitmap format looks more attractive due to fixed size/space requirements.