1. A system, comprising: a monitor computing device that includes a processor and a memory, the memory including instructions executable by the processor, the instructions including instructions for: receiving user input including a command to alter data storage in a cluster, along with parameters for executing the command; obtaining information, from one or more other computing devices, relating to one or more volumes in the cluster and information relating to devices in the cluster; generating, in the monitor computing device, and without receiving further user input, a formal description of a placement function that maps one or more object identifiers to a storage device set based on the information obtained from one or more other computing devices relating to one or more volumes in the cluster and devices in the cluster; generating placement function code by compiling the formal description of the placement function to computer-executable code; and at least one second device that includes a second processor and a second memory, the second memory configured to store the placement function code, and the second processor configured to execute the placement function code.