iSCSI data path requirements

• When multiple remote paths share a port, set the same value of blocked path SIM monitoring time (time for watching SIMs reported when paths are blocked) for all remote paths.
• (VSP One B20 series) When the connection destination of a remote path is a 25-Gbps Ethernet channel board, the DKCMAIN firmware of the connection source must support the connection of a 25-Gbps Ethernet channel board to the remote path. For this reason, even if the connection source is a 10-Gbps iSCSI channel board, do not downgrade the DKCMAIN firmware to a version that does not support the connection of a 25-Gbps Ethernet channel board to a remote path. If you do so, the remote path is blocked.

Physical paths

• To add a remote path to a path group, you should configure the remote paths by using the same protocol. The configuration containing both Fibre Channel and iSCSI connections might impact performance.
• Before replacing Fibre Channel or iSCSI physical paths, remove the GAD pair and the remote path that are using the physical path to be replaced.
• Using the same protocol in the physical paths between the host and the storage system and between storage systems is recommended.

  As in the example below, if protocols are mixed, set the same or a greater command timeout value between the host and the storage system than between storage systems.

  Example:

  • Physical path between the host and a storage system: Fibre Channel
  • Physical path between storage systems: iSCSI
• (VSP One Block 20 series) Using multiple iSCSI paths for the physical paths between storage systems might cause a difference in response time between the physical paths. If you want to suppress the difference in response time, reduce the imbalance in the number of commands running at the same time by shortening the data transfer time. For this reason, specify the following settings for the ports (both MCU and RCU) on the physical paths:
  • Set HMO 104 (iSCSI maximum transfer size expansion mode) to ON.*
  • Set HMO 127 (10Gbps iSCSI Transmit Throttle mode) to ON.*

  * Set these HMOs only when instructed by customer support.

  After specifying these settings, if the difference is not reduced, expand the bandwidth of the paths or add a physical path.

Ports

• When the parameter settings of an iSCSI port are changed, the iSCSI connection is temporarily disconnected and then reconnected. To minimize the impact on the system, change the parameter settings when the I/O load is low.
• If you change the settings of an iSCSI port connected to the host, a log might be output on the host, but this does not indicate a problem. In a system that monitors system logs, an alert might be output. If an alert is output, change the iSCSI port settings, and then check if the host is reconnected.
• When you use an iSCSI interface between storage systems, disable Delayed ACK by using the raidcom modify port -delayed_ack_mode command. By default, Delayed ACK is enabled.

  If Delayed ACK is enabled, it might take time for the host to recognize the volume used by a GAD pair. For example, when the number of volumes is 2,048, it can take approximately 8 minutes. Do not change the default setting (enabled) of Selective ACK for ports.

• In an environment in which a delay occurs in a line between storage systems, such as long-distance connections, you must set an optimal window size of iSCSI ports in storage systems at the primary and secondary sites after verifying various sizes. The window size of the iSCSI port depends on the port used for the remote path:
  • 25-Gbps Ethernet channel board: 64 KB through 2,048 KB (256 KB by default)
  • (VSP One B20 series) 10-Gbps iSCSI channel board: 64 KB through 1,024 KB (64 KB by default)
• iSCSI ports do not support the fragmentation (splitting packets) functionality. When the value for the maximum transfer unit (MTU) of a switch is smaller than the MTU value of the iSCSI port, packets are lost, and communication might not be performed correctly. The MTU value for the iSCSI port must be greater than 1500. Set the same MTU value (or greater) for the switch as the iSCSI port. For more information about the MTU setting and value, see the switch documentation.

  In a WAN environment in which the MTU value is smaller than 1500, fragmented data cannot be sent or received. In this 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 greater.

• iSCSI header digest (a function to protect header information in iSCSI communications from errors) and data digest (a function to protect data information in iSCSI communications from errors) are enabled by default.
  The following restrictions apply:

  • 25-Gbps Ethernet channel board:

    In addition to iSCSI header and data digest, its own protection function is set. If you want to prioritize the performance, disable iSCSI header and data digest to avoid duplication of the protection function.

    For remote paths in one path group, configure the iSCSI header and data digest settings to the same setting (enabled or disabled). If the path group contains remote paths with the settings enabled and disabled, the setting operation cannot be performed.

  • (VSP One Block 20 series) 10-Gbps iSCSI channel board:

    iSCSI header and data digest settings cannot be changed to disabled.

• When using a remote path on the iSCSI port for which virtual port mode is enabled, use the information about the iSCSI port that has virtual port ID (0). You cannot use virtual port IDs other than 0 as a virtual port.
• A port can be used for connections to the host (Target attribute) and to a storage system (Initiator attribute). However, connect separately the port with the Target or Initiator attribute to a different channel board due to the following reasons:
  • If a failure occurs on either the host or the storage system, the scope of impact on the storage system can be reduced.
  • Each target and initiator can perform I/O operations at the maximum data transfer rate of the channel board.
• Use a redundant configuration for remote paths that use ports on the 25-Gbps Ethernet channel board.
• (VSP One Block 20 series) When you configure the remote paths, use the 10-Gbps iSCSI channel board or the 25-Gbps Ethernet channel board for both the MCU and RCU. If the 10-Gbps iSCSI channel board is used for the MCU or RCU and the 25-Gbps Ethernet channel board is used for the other, I/O performance differences might occur depending on the direction of communications.
• A bidirectional remote path is used for the connection between MCU and RCU, and therefore, both the MCU and RCU become targets for the connection between the storage systems.
• You can assign up to 255 iSCSI targets to one port on the 10-Gbps iSCSI channel board (VSP One Block 20 series) or 25-Gbps Ethernet channel board. Up to 255 iSCSI targets can be assigned even if one port contains both targets that connect to the hosts and targets that connect between the storage systems (including targets used for the connections to the external storage systems and the remote copy connections).

  Use the following equation to calculate the number of iSCSI targets assigned to one port:

  number-of-iSCSI-targets = number-of-connections-to-hosts + number-of-ports-on-iSCSI-10-Gbps-channel-board-for-initiator (VSP One Block 20 series) + number-of-ports-on-25-Gbps-Ethernet-channel-board-for-initiator × 4

• You can install 10GBASE-T SFP modules on the 25-Gbps Ethernet channel board ports. When you do so, the transfer speed is 10 Gbps. 25 Gbps is not available. The only difference between a 25-Gbps Ethernet channel board with 10GBASE-T SFP modules and a 25-Gbps Ethernet channel board without 10GBASE-T SFP modules is the transfer speed. There are no differences in other parameters and cautions.

(VSP One Block 85)

(VSP One Block 20 series)

Network setting

• Disable the spanning tree setting for a port on a switch connected to an iSCSI port. If the spanning tree function is enabled on a switch, packets do not loop through a network when the link is up or down. When this happens, packets might be blocked for about 30 seconds. If you need to enable the spanning tree setting, enable the Port Fast function of the switch.
• In a network path between storage systems, if you use a line that has a slower transfer speed than the iSCSI port, packets are lost, and the line quality is degraded. Configure the system so that the transfer speed for the iSCSI ports and the lines is the same.
• Delays in lines between storage systems vary depending on system environments. Validate the system to check the optimal window size of the iSCSI ports in advance. If the impact of the line delay is major, consider using devices for optimizing or accelerating the WAN.
• When iSCSI is used, packets are sent or received using TCP/IP. Because of this, the amount of packets might exceed the capacity of a communication line, or packets might be resent. As a result, performance might be greatly affected. Use Fibre Channel data paths for critical systems that require high performance.