iSCSI requirements and cautions

True Copy User Guide for VSP One Block
Version
10.4.x
Audience
anonymous
Part Number
MK-23VSP1B020-04

For the iSCSI interface, direct, switch, and channel extender connections are supported. The following table lists the requirements and cautions for systems using iSCSI data paths. For details about the iSCSI interface, see the Provisioning Guide for your storage system.

Item

Requirement

iSCSI front-end director

The 8IS10 (10 Gbps) front-end director (FED) is required for remote copy connections.

Remote paths

If iSCSI is used for a remote path, the blocked path monitoring remote replica option must be set to at least 40 seconds (default). If blocked path monitoring is less than 40 seconds, the path might be blocked due to a delay in the network such as many switches in a spanning tree protocol (STP) network or a long distance connection. For instructions, see Setting the remote replication options.

Physical paths

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

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

    Example:

    • Physical path between the host and a storage system: Fibre Channel
    • Physical path between storage systems: iSCSI
  • Using multiple iSCSI paths for the physical paths between DKCs 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:
    • Expand the window size (1,024 KB).
    • 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.

    If the difference is not reduced after specifying these settings, 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 (raidcom modify port -delayed_ack_mode disable). 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 TC pair. For example, when the number of volumes is 2,048, it takes up to 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)
    • 10-Gbps iSCSI channel board: 64 KB through 1,024 KB (64 KB by default)
  • iSCSI ports do not support fragment processing (dividing a packet). When the maximum transmission unit (MTU) of a switch is smaller than that of an iSCSI port, packets might be lost, and data cannot be transferred correctly. The MTU value for the switch must be the same as or greater than the MTU value for the iSCSI port. For details of the MTU setting and value, see the user documentation for the switch.

    The MTU value for the iSCSI port must be greater than 1500. In a WAN environment in which the MTU value is 1500 or smaller, fragmented data cannot be transferred. In this case, lower the maximum segment size (MSS) of the WAN router according to the WAN environment, and then connect to an iSCSI port. Alternatively, use a WAN environment in which the MTU value is greater than 1500.

  • 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.

    • 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 a port with the Target or Initiator attribute to a different channel board for 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.
  • When you configure the remote paths, if the 10-Gbps iSCSI channel board or the 25-Gbps Ethernet channel board is used for either MCU or RCU, use the 25-Gbps Ethernet channel board for RCU. The I/O performance improvements can be expected compared to using the 10-Gbps iSCSI channel board for RCU.
  • A bidirectional remote path is used for the connection between the 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 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 external storage systems and 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-10-Gbps-iSCSI-channel-board-for-initiator + number-of-ports-on-25-Gbps-Ethernet-channel-board-for-initiator × 4

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.