There have been the number of mballoc optimizations, but allocator in general allocates blocks from lower block groups to later in sequential order if the filesystem is free.
It would be nice to have another allocator policy that allows e.g. pseudo-random allocation across block groups in the ldiskfs.

There are some use cases

1. Consistent block allocation performance
   Even peak performance is slow in that policy, it can keep consist performance regardless filesystem is empty and full.
2. Benchmark, test and debug purpose
   In order to optimize mballoc or run benchmark, it would have fragmented filesystem conditions to see how improvements work.
   Today, we are using fallocate (allocate/punch-hole) to make such filesystem conditions. If a random block allocator policy is available, it could help for that debug/benchmark.