Support for an adjustable PathRecoveryInterval in Windows Server 2008

Source: Microsoft Support

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SUMMARY

Windows Server 2008 Service Pack 2 includes new functionality that allows for custom configuration of the path recovery interval used by MPIO. Configuration is controlled through the registry, after adding the following values as REG_DWORDs:

• UseCustomPathRecoveryInterval
• PathRecoveryInterval

These were introduced to account for the following scenario:

• A transient error causes a path to briefly fail and recover.
• MPIO detects that the path has failed and thus performs a fail-over.
• The failed path was the last path for a particular pseudo-LUN, so its PDO Remove Timer starts ticking down.
• The transient error was so brief, that PnP did not register the fact that the path went away and then came back. Thus, there are no PnP events generated to indicate that the path is back online.
• The pseudo-LUN never sees the path come back online and it gets removed once its PDO Remove Timer runs out.

In this scenario, the system now has at least one path and one device online, but no pseudo-LUN to represent that device.

In most cases, the default is applicable, but it does not solve the problem in this particular scenario. That is where the settings above come into play. They allow the user to configure the timer that determines the period at which path recovery attempts are done. Thus, by setting the PathRecoveryInterval to less than the PdoRemovePeriod, the path recovery attempt will happen before the pseudo-LUN gets removed, the path will be detected as back online, and the pseudo-LUN can be saved from removal.

Caution is advised when setting the PathRecoveryInterval to small values. By decreasing this value, larger amounts of path verification traffic will be generated. This traffic will increase with the number of LUNs available on the host, and the smaller the value. It is recommended that use of this value be tested before widespread deployment in production to ensure that path recovery attempts are not happening so frequently that it has a significant impact on regular I/O.

For example, if PDORemovePeriod is set to 60 seconds, a good starting point for PathRecoveryInterval may be 30 seconds. This would cause path recovery to be attempted every 30 seconds.

There are two registry subkeys involved in this new timer (both are DWORDs):

• HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\mpio\Parameters\UseCustomPathRecoveryInterval
• HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\mpio\Parameters\PathRecoveryInterval

If both the UseCustomPathRecoveryInterval value and the PathRecoveryInterval value are non-zero, then the MPIO driver's internal path recovery interval is set to the PathRecoveryInterval from the registry.

If either the UseCustomPathRecoveryInterval value or the PathRecoveryInterval value is zero, the driver defaults to the behavior where PathRecoveryInterval is twice that of PDORemovePeriod.

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