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IBM FlashSystem 9100 implements the concept of each volume having a preferred owner node, which improves cache efficiency and cache usage. The cache component read/write algorithms depend on one node that owns all the blocks for a specific track. The preferred node is set at the time of volume creation manually by the user or automatically by IBM FlashSystem 9100. 

Because read-miss performance is better when the host issues a read request to the owning node, you want the host to know which node owns a track. The SCSI command set provides a mechanism for determining a preferred path to a specific volume. Because a track is part of a volume, the cache component distributes ownership by volume. The preferred paths are then all the paths through the owning node. Therefore, a preferred path is any port on a preferred controller, assuming that the SAN zoning is correct. 

Tip:  Performance can be better if the access is made on the preferred node. The data can still be accessed by the partner node in the I/O group if a failure occurs. 

By default, IBM FlashSystem 9100 assigns ownership of even-numbered volumes to one node of a caching pair and the ownership of odd-numbered volumes to the other node. It is possible for the ownership distribution in a caching pair to become unbalanced if volume sizes are different between the nodes or if the volume numbers that are assigned to the caching pair are predominantly even or odd. 

To provide flexibility in making plans to avoid this problem, the ownership for a specific volume can be explicitly assigned to a specific node when the volume is created. If a node becomes overloaded, the preferred node of a volume can be changed to the other node in the same I/O group, or to a node in another I/O group. This procedure can be performed concurrently with I/O operations if the host supports non-disruptive volume move. Figure 5-16 shows a write operation from a host. 

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