IBM DS8000 manual Determining the number of paths to a LUN, Determining where to attach the host

12.4.6 Determining the number of paths to a LUN

When configuring the IBM DS8000 for an open systems host, a decision must be made regarding the number of paths to a particular LUN, because the multipathing software allows (and manages) multiple paths to a LUN. There are two opposing factors to consider when deciding on the number of paths to a LUN:

￿Increasing the number of paths increases availability of the data, protecting against outages.

￿Increasing the number of paths increases the amount of CPU used because the multipathing software must choose among all available paths each time an I/O is issued.

A good compromise is between 2 and 4 paths per LUN.

Subsystem Device Driver (SDD): Dynamic I/O load-balancing

The Subsystem Device Driver is a pseudo device driver designed to support the multipath configuration environments in the IBM TotalStorage DS8000. It resides in a host system with the native disk device driver as described in Chapter 15, “Open systems support and software” on page 319.

The dynamic I/O load-balancing option (default) of SDD is recommended to ensure better performance because:

￿SDD automatically adjusts data routing for optimum performance. Multipath load balancing of data flow prevents a single path from becoming overloaded, causing input/output congestion that occurs when many I/O operations are directed to common devices along the same input/output path.

￿The path to use for an I/O operation is chosen by estimating the load on each adapter to which each path is attached. The load is a function of the number of I/O operations currently in process. If multiple paths have the same load, a path is chosen at random from those paths.

12.4.7Determining where to attach the host

When determining where to attach multiple paths from a single host system to I/O ports on a host adapter to the storage facility image, the following considerations apply:

￿Choose the attached I/O ports on different host adapters.

￿Spread the attached I/O ports evenly between the four I/O enclosure groups.

￿Spread the I/O ports evenly between the different RIO-G loops.

The DS8000 host adapters have no server affinity, but the device adapters and the rank have server affinity, as illustrated in Figure 12-11 on page 269.

A host is connected through two FC adapters to two DS8000 host adapters located in different I/O enclosures. The host has access to LUN1, which is created in the extent pool 1 controlled by the DS8000 server 0. The host system sends read commands to the storage server. When a read command is executed, one or more logical blocks are transferred from the selected logical drive through a host adapter over an I/O interface to a host.

In the following case the logical device is managed by server 0 and the data is handled by server 0 as shown in Figure 12-11.The read data to be transferred to the host must first be present in server 0's cache. When the data is in the cache it is then transferred through the host adapters to the host.

268DS8000 Series: Concepts and Architecture