Determining the size of the shared memory pool

You need to consider the degree to which you want to overcommit the physical memory in the shared memory pool, the performance of the workloads when running in a shared memory configuration that is overcommitted, and the minimum and maximum boundaries of the shared memory pool.

1. Consider the degree to which you want to overcommit the physical memory in the shared memory pool.
   • When the sum of the physical memory currently used by the shared memory partitions is less than or equal to the amount of memory in the shared memory pool, the memory configuration is logically overcommitted. In a logically overcommitted memory configuration, the shared memory pool has enough physical memory to contain the memory used by all shared memory partitions at one point in time.
   • When the sum of the physical memory currently used by the shared memory partitions is greater than the amount of memory in the shared memory pool, the memory configuration is physically overcommitted. In a physically overcommitted memory configuration, the shared memory pool does not have enough physical memory to contain the memory used by all the shared memory partitions at one point in time. The hypervisor stores the difference in auxiliary storage.
2. Consider the performance of the workloads when running in a shared memory configuration that is overcommitted. Some workloads perform well in a shared memory configuration that is logically overcommitted, and some workloads can perform well in a shared memory configuration that is physically overcommitted.
   Tip: In general, more workloads perform better in logically overcommitted configurations than physically overcommitted configurations. Consider limiting the degree to which you physically overcommit the shared memory pool.
3. The shared memory pool must be large enough to meet the following requirements:
   1. The shared memory pool must be large enough to provide each shared memory partition with its I/O entitled memory when all of the shared memory partitions are active. When you create a shared memory partition, the Hardware Management Console (HMC) and the Integrated Virtualization Manager (IVM) automatically determine the I/O entitled memory for the shared memory partition. After you activate the shared memory partitions, you can view statistics about how the operating systems use their I/O entitled memory and adjust the I/O entitled memory of the shared memory partitions accordingly.
   2. A small portion of the physical memory in the shared memory pool is reserved for the hypervisor so that it can manage shared memory resources. The hypervisor requires: (a small amount of physical memory per shared memory partition) + 256 MB.
   Tip: To ensure that you can successfully activate the shared memory partitions, assign at least the following amount of physical memory to the shared memory pool: (the sum of the minimum logical memory that is assigned to all of the shared memory partitions that you plan to run concurrently) + (the required 256 MB of reserved firmware memory).
4. When the shared memory pool is equal to or greater than the sum of the assigned logical memory of all the shared memory partitions plus the required amount of reserved firmware memory, the initial shared memory configuration is not overcommitted. Therefore, the amount of physical memory that you assign to the shared memory pool need not exceed the sum of the assigned logical memory of all the shared memory partitions plus the required amount of reserved firmware memory.