Preparing to configure shared memory on a system that is managed by an HMC

Before you configure the shared memory pool and create logical partitions that use shared memory (hereafter referred to as shared memory partitions), you need to determine the size of the shared memory pool, the amount of memory to assign to each shared memory partition, the number of paging space devices to assign to the shared memory pool, and the redundancy configuration of the Virtual I/O Server logical partitions that you assign to the shared memory pool.

1. Assess your needs, take inventory of your current environment, and plan for capacity. For instructions, see Planning for logical partitions. More specifically, determine the following information:
   1. Determine the number of shared memory partitions to assign to the shared memory pool.
   2. Determine the amount of logical memory to assign as the desired, minimum, and maximum logical memory for each shared memory partition. You can apply the same general guidelines that you might use to assign the desired, minimum, and maximum dedicated memory to logical partitions that use dedicated memory. For example:
      • Do not assign the maximum logical memory to a value that is higher than the amount of logical memory that you plan to dynamically add to the shared memory partition.
      • Set the minimum logical memory to a value that is high enough for the shared memory partition to successfully activate.
2. Determine the amount of physical memory to assign to the shared memory pool. For instructions, see Determining the size of the shared memory pool.
3. Prepare for paging space devices:
   1. Determine the number of paging space devices to assign to the shared memory pool. The HMC assigns one paging space device to each shared memory partition that is active. Thus, the fewest number of paging space devices that must be assigned to the shared memory pool is equal to the number of shared memory partitions that you plan to run simultaneously. For example, you assign ten shared memory partitions to the shared memory pool and you plan to run eight of the shared memory partitions simultaneously. Thus, you assign at least eight paging space devices to the shared memory pool.
   2. Determine the size of each paging space device:
      • For AIX® and Linux Linux shared memory partitions, the paging space device must be at least the size of the maximum logical memory size of the shared memory partition that you identified in step 1b. For example, you plan to create an AIX shared memory partition with a maximum logical memory size of 16 GB. The paging space device must be at least 16 GB.
      • For IBM® i shared memory partitions, the paging space device must be the size of the maximum logical memory size of the shared memory partition that you identified in step 1b multiplied by 129/128. For example, you plan to create an IBM i shared memory partition with a maximum logical memory size of 16 GB. The paging space device must be at least 16.125 GB.
      • Consider creating paging space devices that are large enough to be used by shared memory partitions with multiple partition profiles.
   3. Determine whether each paging space device resides in physical storage on the server or on a storage area network (SAN). Paging space devices that are accessed by a single Virtual I/O Server (VIOS) logical partition (hereafter referred to as a paging VIOS partition) can be located in physical storage on the server or on a SAN. Paging space devices that are redundantly accessed by two paging VIOS partitions must be located on a SAN.
4. Prepare for paging VIOS partitions:
   1. Determine which Virtual I/O Server (VIOS) logical partitions can be assigned to the shared memory pool as paging VIOS partitions. A paging VIOS partition provides access to the paging space devices for the shared memory partitions that are assigned to the shared memory pool. A paging VIOS partition can be any active Virtual I/O Server (version 2.1.1, or later) that has access to the paging space devices that you plan to assign to the shared memory pool.
   2. Determine the number of paging VIOS partitions to assign to the shared memory pool. You can assign one or two paging VIOS partitions to the shared memory pool:
      • When you assign a single paging VIOS partition to the shared memory pool, it must have access to all of the paging space devices that you plan to assign to the shared memory pool.
      • When you assign two paging VIOS partitions to the shared memory pool, each paging space device that you plan to assign to the shared memory pool must be accessible to at least one paging VIOS partition. However, usually when you assign two paging VIOS partitions to the shared memory pool, they redundantly access one or more paging space devices.
   3. If you plan to assign two paging VIOS partitions to the shared memory pool, determine how you want to configure redundancy for the shared memory partitions:
      1. Determine which shared memory partitions to configure to use redundant paging VIOS partitions. For each shared memory partition, this means that both paging VIOS partitions can access the shared memory partition's paging space device.
      2. Determine which paging VIOS partition to assign as the primary paging VIOS partition and which paging VIOS partition to assign as the secondary paging VIOS partition for each shared memory partition. The hypervisor uses the primary paging VIOS partition to access the paging space device that is assigned to the shared memory partition. If the primary VIOS partition becomes unavailable, the hypervisor uses the secondary paging VIOS partition to access the paging space device that is assigned to the shared memory partition.
5. Determine the number of additional processor resources that are needed for the paging VIOS partitions. To read and write data between the paging space devices and the shared memory pool, the paging VIOS partitions require additional processing resources. The amount of additional processing resources that are needed depends on the frequency that the paging VIOS partition reads and writes the data. The more frequently that the paging VIOS partition reads and writes the data, the more frequently the paging VIOS partition performs I/O operations. More I/O operations require more processing power. In general, the frequency that the paging VIOS partition reads and writes data can be affected by the following factors:
   • The degree to which the shared memory partitions are overcommitted. In general, greatly overcommitted shared memory partitions require the paging VIOS partition to read and write data more often than slightly overcommitted shared memory partitions.
   • The I/O rates of the storage subsystem on which the paging space devices are located. In general, paging space devices with faster I/O rates (such as a SAN) enable the paging VIOS partition to read and write data more often than paging space devices with slower I/O rates (such as storage in the server).
   You can use the IBM Systems Workload Estimator (WLE) to determine the number of processor resources that are needed for paging VIOS partitions.