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iSCSI
front-end director
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The 8IS10 (10 Gbps)
front-end director (FED) is required for remote copy connections.
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Remote paths
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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.
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Physical paths
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- 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
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Ports
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- 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 (Edit Ports window in
HDvM - SN or
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 maximum value you can set is 1,024 KB. The default window size is 64 KB, so you must change this setting.
- 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.
- 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.
- On VSP Gx00 models and VSP Fx00 models, and VSP E series, a port can be used for
connections to the host (target attribute) and to a storage
system (initiator attribute). However, to minimize the
impact on the system, if a failure occurs either on the host
or in a storage system, you should connect the port for the
host and for the storage system to separate CHBs.
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Network setting
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- 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.
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