If any Ubiblk installations are enabled on a VmHost, the allocator will
choose one at random, weighted by `allocation_weight`. Otherwise, SPDK
will be used.
Currently, Ubiblk is not installed by default and must be set up
manually by an operator. Therefore, the default behavior in production
is to continue using SPDK.
Note: The values for `num_queues` and `queue_size` are provisional and
may change based on future performance and resource usage experiments.
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