HiCommand Dynamic Link Manager User's Guide (for Windows(R))

2.7.1 Automatic path switching

The following describes the automatic failover and automatic failback functionalities that automatically switch a path.

Organization of this subsection

(1) Automatic failover

(2) Automatic failback

(1) Automatic failover

If you detect an error for a path in use, you can keep operating the system by changing the state to offline, and using other online paths. This functionality is called automatic failover. Automatic failover is applicable to the following levels of errors that occur on a path.

Critical

A fatal error that may stop the system.

Error

A high-risk error that can be avoided by using failover or other countermeasures.

For details on error levels, see 2.11.2 Filtering of error information.

When the Thunder 9200, Thunder 9500V Series, or TagmaStore AMS/WMS series is being used, HDLM selects the switching destination from the paths that access the same LU, in the order of owner paths and non-owner paths. For example, in Figure 2-10 Path switching, the owner controller of LU is CHA0, and access to the LU is made only via the path (A). After the access path is placed offline, the first candidate for the switching destination is the path (B) and the second candidate is the path (C) or (D).

When the Lightning 9900 Series, Lightning 9900V Series, TagmaStore USP, or Universal Storage Platform V is being used, all the paths are owner paths. This means all the paths accessing the same LU can be possible switching destinations. For example, in Figure 2-10 Path switching, the LU is accessed using only the path (A). After the access path is placed offline, the switching destination is one of the paths (B), (C), and (D).

Figure 2-10 Path switching

(2) Automatic failback

After a path recovers from an error, HDLM can automatically place the recovered path online. This functionality is called automatic failback.

When using this function, HDLM monitors error recovery on a regular basis.

When using the Thunder 9200, Thunder 9500V Series, or TagmaStore AMS/WMS series, HDLM selects the path to use from online owner paths, and then from online non-owner paths. Therefore, if an owner path recovers from an error and HDLM automatically places the recovered path online while any non-owner path is in use, the path to use is switched to the owner path.

When the Lightning 9900 Series, Lightning 9900V Series, TagmaStore USP, or Universal Storage Platform V is being used, all the paths are owner paths. Therefore, if an owner path recovers from an error and HDLM automatically places the recovered path online, the path to use is not switched.

When intermittent errors^# occur in paths, the path status may frequently alternate between the online and offline status if you are using automatic failback, so the I/O performance might deteriorate. In such a case, if there is a path in which an intermittent error might be occurring, we recommend that you set up intermittent error monitoring to remove that path from those subject to automatic failback.

You can specify the automatic failback function or intermittent error in the Options window of the HDLM GUI or by the dlnkmgr command's set operation. For details on the operation in the Options window, see 4.2.8 Viewing or setting up the operating environment and 6.3 Options window. For details on the set operation, see 7.6 set (Sets the operating environment).

#

An intermittent error means an error that occurs irregularly because of some reason such as a loose cable connection.

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