As has been explained in a recent email to Mira users, the minimum partition you can use on the machine is 512 nodes. If you request fewer nodes, you still pay from your allocation for all 512, and the unused nodes are idle. On Cetus, the minimum partition size is 128 nodes.
As some of you exhaust your ESP allocations on Mira, you will notice your jobs going into the “backfill” queue. These are queued with low priority relative to positive-allocation-balance jobs, but will run if resources are available and no normal jobs are available to fit the space. The maximim size job allowed in backfill mode is 8192 nodes.