Currently the LBM allocates across disks in round-robin order. I ran a test using 4 data disks and a table with 12 columns. This caused very uneven allocation across disks, since all data for column N ended up on disk N%4. This also makes single-column scans slow since they read from only a single disk.
In a more mixed workload this is likely to be less problematic, but still it seems like we should take available space into consideration for allocation, and/or add more randomness.