Oversubscription at migration zone level

Oversubscription at virtual data center creation time is calculated by category across a migration zone. Virtual data centers are allocated inside migration zones, and oversubscription is calculated after the virtual data centers are created.

For example: consider we have three nodes in a migration zone, each with 20 cores and 128 GB of memory. Node 1 and Node 2 are standard nodes (category = General) and Node 3 is a high-performance node (category = High_Perf). In total, we have 40 cores and 256 GB of General and 20 cores and 128 GB of High_Perf available. Virtual data centers are allocated from migration zones and storage pools, and application instances are provisioned inside virtual data centers.

Consider that we now create three virtual data centers from the three nodes in the migration zone, as shown in the table below. VDC1 is for development and test, VDC2 is for production and VDC3 is for DevOps. VDC1 is created with 16 cores and 160 GB of RAM of the General category, adequate for dev/test needs. VDC2 is created with 24 cores and 96 GB of General, and 24 cores and 160 GB of High_Perf. Some of the production application instances need nodes from the High_Perf category; others see the General nodes as adequate. Finally, VDC3 is created with 20 cores and 128 GB of General and 4 cores and 12 GB of High_Perf. In total, we have allocated 60 cores and 384 GB of General and 28 cores and 172 GB of High_Perf. The subscription factors for both cores and memory for the General category are 1.5. The subscription factors for the High_Perf category are 28/20 or 1.4 for cores, and 172/128 or 1.34 for memory. These subscription factors change if we allocate a new virtual data center, or if one or more of the existing virtual data centers change the number of cores or memory allocated to them. We use the term “oversubscribed” to refer to a situation when the subscription factor exceeds 1 (or the subscription percentage exceeds 100%).

It is useful to re-emphasize two points about the definition of subscription factors at allocation time. First, they do not apply to individual nodes; they apply to all nodes of a migration zone that are in a specific category. Second, the subscription factor is based on virtual datacenter allocations, not on application instances we have provisioned. In the above example, the High_Perf and General categories become oversubscribed as soon as the virtual data centers are allocated. Even if we have not yet started a single application in these virtual data centers, these categories can become oversubscribed, as the above example demonstrates.

Table 1. Virtual data center example
 

GENERAL Category

HIGH_PERF Category

VDC1 (Dev/Test)

16 cores, 160 GB

 

VDC2 (Production)

24 cores, 96 GB

24 cores, 160 GB

VDC3 (DevOps)

20 cores, 128 GB

4 cores, 12 GB

TOTAL in virtual data centers

60 cores, 384 GB

28 cores, 172 GB

TOTAL AVAILABLE

40 cores, 256 GB

20 cores, 128 GB

Subscription Factor

1.5 CPU, 1.5 RAM

1.4 CPU, 1.34 RAM