8456ch_RAS.fm Draft Document for Review January 13, 2020 5:02 pm IBM Z 3.3.3 Metadata checks 80  IBM DS8900F Architecture and Implementation In the IBM Z environment, the preferred practice is to provide multiple paths from each host to  a storage system. Typically, four or eight paths are installed. The channels in each host that  can access each logical control unit (LCU) in the DS8900F are defined in the hardware  configuration definition (HCD) or input/output configuration data set (IOCDS) for that host.  Dynamic Path Selection (DPS) allows the channel subsystem to select any available  (non-busy) path to start an operation to the disk subsystem. Dynamic Path Reconnect (DPR)  allows the DS8900F to select any available path to a host to reconnect and resume a  disconnected operation, for example, to transfer data after disconnection because of a cache  miss. These functions are part of the IBM Z architecture, and are managed by the channel  subsystem on the host and the DS8900F.  A physical FICON path is established when the DS8900F port sees light on the fiber, for  example, a cable is plugged in to a DS8900F host adapter, a processor or the DS8900F is  powered on, or a path is configured online by z/OS. Now, logical paths are established  through the port between the host, and part or all of the LCUs in the DS8900F are controlled  by the HCD definition for that host. This configuration happens for each physical path  between a IBM Z host and the DS8900F. Multiple system images can be in a CPU. Logical  paths are established for each system image. The DS8900F then knows the paths that can  be used to communicate between each LCU and each host. Control-unit initiated reconfiguration (CUIR) varies off a path or paths to all IBM Z hosts to  allow service to an I/O enclosure or host adapter, then varies on the paths to all host systems  when the host adapter ports are available. This function automates channel path  management in IBM Z environments in support of selected DS8900F service actions. CUIR is available for the DS8900F when it operates in the z/OS and IBM z/VMfi  environments. CUIR provides automatic channel path vary on and vary off actions to  minimize manual operator intervention during selected DS8900F service actions. CUIR also allows the DS8900F to request that all attached system images set all paths that  are required for a particular service action to the offline state. System images with the correct  level of software support respond to such requests by varying off the affected paths, and  either notifying the DS8900F subsystem that the paths are offline, or that it cannot take the  paths offline. CUIR reduces manual operator intervention and the possibility of human error  during maintenance actions, and reduces the time that is required for the maintenance. This  function is useful in environments in which many z/OS or z/VM systems are attached to a  DS8900F. When application data enters the DS8900F, special codes or  metadata , which is also known  as  redundancy checks , are appended to that data. This metadata remains associated with the  application data while it is transferred throughout the DS8900F. The metadata is checked by  various internal components to validate the integrity of the data as the data moves throughout  the disk system. It is also checked by the DS8900F before the data is sent to the host in  response to a read I/O request. The metadata also contains information that is used as an  extra level of verification to confirm that the data that is returned to the host is coming from the  location that you want on the disk.  The metadata check is independent of the DS8900F T10 DIF support for fixed-block volumes.  For more information about T10 DIF implementation in the DS8880, see “T10 data integrity  field support” on page 109. 