Before configuring a system that uses iSCSI, review the following considerations. For details about iSCSI, see the Provisioning Guide. For other considerations when using Universal Volume Manager, see Planning considerations for external storage systems.
Considerations for external paths
- (VSP 5000 series) The external volumes that use iSCSI for the external paths cannot be accessed from a mainframe operating system. (The emulation types such as 3390-x cannot be set for the external volumes.)
- When adding an external path to a path group, make sure that it uses the same protocol as the other paths in the group. External paths for Fibre Channel and iSCSI cannot exist in the same path group.
- If iSCSI is used for an external path,
set Blocked Path
Monitoring to at least 40 seconds (default). If you set a lower
value, the external path might be blocked due to network delays, including the
spanning tree functionality of a switch.
In addition, if external volumes are accessed from a host, set the command timeout period of the host to a value greater than the value of Blocked Path Monitoring. Otherwise, commands from the host might time out when the external storage system is turned off or a failure occurs.
- After adding an iSCSI path, execute the login test for the iSCSI target to check if you can log in. If there is an iSCSI path from which you cannot log in to the iSCSI target, attempts to connect to the path are made, and the load to storage systems or networks may become high. If this happens, external volumes might not be recognized.
- For an iSCSI path from which you cannot log in to the target, edit the iSCSI target or check settings of the external storage system to make sure you can log in. Alternatively, remove the iSCSI path.
Considerations for data paths
- When replacing Fibre Channel or iSCSI data paths, first remove any external paths that use the data paths to be replaced.
- Use the same protocol for data paths between a host and a storage system, or between storage systems.
- When Fibre Channel is used in a data path between a host and a storage system, and iSCSI is used for a data path between storage systems, different protocols are used in the data path. In this case, set a value which is equal to or greater than the timeout period for the command between storage systems as the timeout period of the command between the host and the storage system.
Considerations for ports
- When you change parameter settings for an iSCSI port, the iSCSI connection is temporarily disconnected, and then reconnected. Therefore, you should change parameter settings when the I/O load is low to reduce impact on the system.
- When you change settings for an iSCSI port connected to a host, log information might be output to the host. However, this does not indicate a failure. In a system that monitors system logs, if an alert is issued, change the iSCSI port settings, and then check if the host is reconnected.
- For the iSCSI port connecting to
storage systems or to the host, set Delayed ACK to
Disable.
If Delayed ACK is set to the default value Enable, the host might take a long time to recognize external volumes. For example, 2,048 volumes might take 8 minutes.
- In the Edit Ports window, Selective ACK is set to Enable by default. Do not change this setting.
- In an environment in which a delay occurs in a line between storage systems, such as long-distance connections, try various sizes, and then set an optimal window size of iSCSI ports in storage systems at the primary and secondary sites. The maximum value you can set is 1,024 KB. Note that the default window size is 64 KB.
- In Universal Volume Manager, an external path connection is established for each iSCSI target of an external storage system. The maximum number of iSCSI external paths for each port is 512. However, best practice is to set no more than 127 external paths for each port.
- iSCSI ports do not support the
fragmentation (splitting packets) functionality. If the maximum transfer unit
(MTU) of a switch is smaller than the MTU of the iSCSI port, packets are lost
and communication might not be performed correctly. To avoid this problem, set
the switch MTU to a value equal to or greater than the iSCSI port MTU. Note that
the iSCSI port MTU must be at least 1500.
In a WAN environment in which the MTU value is smaller than 1500, fragmented data cannot be sent or received. In such environment, set a smaller value for the maximum segment size (MSS) of the WAN router according to the WAN environment, and then connect the iSCSI port. Alternatively, use iSCSI in an environment in which the MTU value is 1500 or higher. For details about MTU values for the switch and how to set them, see the documentation for the switch.
- To use Universal Volume Manager on iSCSI ports for which the virtual port mode is enabled, you must use CCI. Additionally, you must use CCI to enable the virtual port mode. Do not set external paths from multiple virtual ports associated with a single iSCSI port of the local storage system to the same iSCSI port of an external storage system. Best practice is to use different iSCSI ports for paths to external storage systems and their alternate paths.
- (VSP E series) A single port can be used for connections to hosts (target attribute) and storage systems (initiator attribute). However, a port on the 25 Gbps iSCSI channel board cannot be used for the iSCSI connection between the storage systems. To minimize the influence on the system if a failure occurs either in a host or storage system, we recommend that you connect ports for hosts and storage systems to separate CHBs.
Notes on network settings
- Best practice is to disable the spanning tree setting on the port of a switch connecting to an iSCSI port. If you enable the spanning tree functionality of a switch, packets might not be looped in the network when the link is up or down. If this happens, the packets might be blocked for approximately 30 seconds. If you must enable the spanning tree settings, enable the port fast functionality of the switch.
- In a network path between storage systems, if you use a line whose transfer speed is slower than the iSCSI port, packets are lost and the line quality is degraded. To avoid this problem, configure the system so that the transfer speed for iSCSI ports and lines is the same.
- The delay in the line between storage systems varies depending on the system environment. Therefore, validate the system first, and then check the optimum window size settings of iSCSI ports. If the influence of the line delay is unacceptable, consider using devices for optimizing or accelerating the WAN speed.
- If you use iSCSI, packets are sent or received using TCP/IP. Therefore, the number of packets might exceed the capacity of the communication line, or packets might be resent. As a result, performance might be greatly affected. Therefore, in critical systems that rely on performance, use Fibre Channel.
- If the external storage system is in the HUS 100 series, the number of iSCSI targets you can search for is limited. If the iSCSI target name is 47 characters (default), you can search up to 170 iSCSI targets.