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You'll often see messages like this:
2012-06-04 17:34:52,558 DEBUG \ [cloud.deploy.FirstFitPlanner\] (Job-Executor-9:job-9) Cannot allocate cluster 1 for vm creation since its allocated percentage: 0.9243709585329766 will cross the disable capacity threshold: 0.85 for capacity Type : 0, skipping this cluster Wiki Markup
The important thing above is: first of all it's noting that you don't have enough resources, but which resource do you not have enough of? The Type, section (highlighted in red) combined with the below table can tell you which resource you are running low on.
CAPACITY_TYPE_MEMORY = 0;
CAPACITY_TYPE_CPU = 1;
CAPACITY_TYPE_STORAGE = 2;
CAPACITY_TYPE_STORAGE_ALLOCATED = 3;
CAPACITY_TYPE_VIRTUAL_NETWORK_PUBLIC_IP = 4;
CAPACITY_TYPE_PRIVATE_IP = 5;
CAPACITY_TYPE_SECONDARY_STORAGE = 6;
CAPACITY_TYPE_VLAN = 7;
CAPACITY_TYPE_DIRECT_ATTACHED_PUBLIC_IP = 8;
CAPACITY_TYPE_LOCAL_STORAGE = 9;
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