Today we just use the metrics/solr.node/CONTAINER.fs.usableSpace metric which returns the usableSpace of the coreRootDirectory. But it has the following problems:
- The solrconfig.xml of the collection's configset may specify a different path than coreRootDirectory
- If solr.data.home is specified as a system property on the node then we must use that directory's usableSpace
- For both of the above, we must also use the coreRootDirectory's usableSpace for correctness otherwise the node will flat out refuse to create a core because it cannot create a core.properties on disk.