Requirements and planning

You can perform Universal Volume Manager (UVM) operations between the local storage system and volumes in an external storage system.

The following table lists the general requirements for all UVM components.

External volumes must meet the Universal Volume Manager requirements.

When planning external volumes, note the following:

• You can use existing data in an external volume after it is mapped to the local storage system, with the following restrictions:

  • To perform host I/O operations, you must set an LU path from the target port to the mapped volume.
  • An external volume cannot be mapped to a DP Pool that has DP-VOLs with the capacity saving function enabled. Capacity saving features are not supported for external storage.
  • You cannot enable the capacity saving function on a DP-VOL from a DP Pool that has pool volumes from external storage.
• The maximum or minimum available capacity of an external volume depends on the emulation type you specify when mapping the volume.
• You cannot access data stored in an external volume that exceeds the allowed maximum capacity. See the Maximum capacity of an external volume row in System requirements .

• Make sure that a mapped external volume is accessed only from the local storage system.

  • Make sure that a mapped external volume is not accessed from a host that is connected to the external storage system.
  • Make sure that a mapped external volume is not manipulated by a copy function or any other functions of the external storage system.
  • Accessing a mapped external volume from the external storage system requires that the volume mapping be disconnected first.
• External volumes that are reserved by a host cannot be mapped as internal volumes. To map these volumes, cancel the reserve settings, remove host access to the volumes, and then perform the mapping operation.
• Do not map multi-platform volumes of external storage systems as internal volumes.
• If an external storage system that uses control unit path ownership is connected to the local storage system, configure the external path to the primary controller in the external storage system as the primary path.

  Ownership is the exclusive right to control volumes. A controller that has ownership is called a primary controller. If the external path is connected to a controller that does not have ownership, and the path is configured as primary path, the ownership will be transferred, which might affect performance.

• A management LU cannot be used as an external volume.

  A management LU receives commands from an application, because it controls or manages the application, and stores control information from the application. An example of a management LU is a Universal Xport LU. (A CCI command device is not a management LU.)

  Before performing the external volume mapping operation, perform one of the following operations on the external storage system:

  • Delete the management LU from the port to be connected to the local storage system.
  • Make sure that at least one LU is used for data storage and has a smaller LUN (LU number) than the management LU's LUN. Also make sure that the data storage LU is set to the port connected to the local storage system.
  • Use the security function and configure the access attribute of the management LU to prohibit read and write operations.

  An external storage system that has a management LU might not be recognized by the local storage system.

The following requirements apply to only to VSP Gx00 models and VSP Fx00 models:

• You cannot map an external volume whose capacity is smaller than the minimum capacity required for internal volumes.

• Before the settings of an external storage system are changed, mapping to volumes on the local storage system must be removed by selecting Disconnect External Volumes. After the external storage system settings are changed, volumes can be remapped. Unless external volumes are remapped, they cannot be used on the local storage system.

• If an LU path is configured on an internal volume mapped to an external volume, the external volume mapping cannot be deleted.

• Mapping cannot be deleted for a volume used in a pair for the following programs:

  • TrueCopy
  • Universal Replicator
  • ShadowImage
  • Thin Image
  • Global-active device

• Mapping cannot be deleted for an external volume configured as a pool volume.

• Mapping cannot be deleted for an external volume configured as a quorum disk.

• All the external volume attributes of an LDEV created within the external volume are the same. These attributes persist even when an LDEV is recreated with the VLL function.
• All LDEVs within an external volume have the same cache mode.
• Attributes, such as port or LUN security, set for a volume on the external storage system side persist when the volume is mapped.
• Mapped volume settings can be made on the local storage system side as necessary.
• Volumes for which the T10 P1 attribute is enabled cannot be specified as an external volume.

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.

• 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.

• 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.

• 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, we recommend trying various sizes, and then setting 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, we recommend you 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. We recommend that you use different iSCSI ports for paths to external storage systems and their alternate paths.

• A single port can be used for connections to hosts (target attribute) and storage systems (initiator attribute). However, to minimize the influence to 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.

• We recommend that you 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.

When configuring a system using Fibre Channels, you need to consider the following restrictions. For details about Fibre Channels, see the Provisioning Guide.

• When you use Fibre Channel as an external path, if you specify Auto for Port Speed, specify 10 seconds or more for Blocked Path Monitoring. If you want to specify 9 seconds or less, do not set Auto for Port Speed.
• If the time specified for Blocked Path Monitoring is not long enough, the network speed might be slowed down or the period for speed negotiation might be exceeded. As a result, paths might be blocked.