Description

Veritas does not recommend clearing the STANDBY flag unless for testing purposes, especially in connection with VPLEX & SRDF/METRO type configurations.

This article outlines how to manually clear the STANDBY flag for DMP path(s) which have reported a high I/O latency. The underlying root cause which is causing the high I/O latency should be addressed.


NOTE: The "STANDBY" flag is only set during boot time & stored in a kernel structure for each enclosure.


Sample output

# vxdmpadm getsubpaths ctlr=c16
NAME         STATE[A]   PATH-TYPE[M] DMPNODENAME  ENCLR-TYPE   ENCLR-NAME     ATTRS        PRIORITY
===================================================================================================
sdp          ENABLED(A)     -          emc0_129e    EMC          emc0            -         -
sdu          ENABLED        -          emc0_129e    EMC          emc0         STANDBY      -
sdq          ENABLED(A)     -          emc0_129f    EMC          emc0            -         -
sdv          ENABLED        -          emc0_129f    EMC          emc0         STANDBY      -
sdr          ENABLED(A)     -          emc0_12a0    EMC          emc0            -         -
sdw          ENABLED        -          emc0_12a0    EMC          emc0         STANDBY      -
sds          ENABLED(A)     -          emc0_12a1    EMC          emc0            -         -
sdx          ENABLED        -          emc0_12a1    EMC          emc0         STANDBY      -
sdt          ENABLED(A)     -          emc0_12a2    EMC          emc0            -         -
sdy          ENABLED        -          emc0_12a2    EMC          emc0         STANDBY      -

The DMP "STANDBY" flag is set for remotely attached EMC array paths where the DMP "latency_threshold_difference" tunable value is reached.  The default value of the latency_threshold_difference tunable parameter is 200 µs. 
 

NOTE: DMP will serve I/O to the local paths only where the remote paths have higher I/O latency and 
are marked with the "STANDBY" flag.
 

How to clear STANDBY flag

Whilst the "vxdmpadm setattr path" command can be used to change the standby (failover) path state to an active path.

NOTE: Caution should be taken when doing so, as performance impact can be observed when reviving the slower "STANDBY" paths to an "ACTIVE" state.

Sample syntax.

# vxdmpadm setattr path sdu pathtype=active

The "STANDBY" flag can be manually set for a given path as follows:

# vxdmpadm setattr path sdu pathtype=standby

The above syntax marks the "sdu" path with a "STANDBY" state and is not used for normal I/O scheduling. This path is only used if there are no active paths available for I/O. 

Whilst the "STANDBY" flag state can be toggled on and off, the flag may reappear if the I/O latency still remains higher than the defined "latency_threshold_difference" value for the specific enclosure.

Sample output

# vxdmpadm getattr enclosure emc0
ENCLR_NAME      ATTR_NAME                     DEFAULT        CURRENT
============================================================================
emc0           iopolicy                      MinimumQ       MinimumQ
emc0           partitionsize                 512            512           
emc0           use_all_paths                 -              -             
emc0           recoveryoption[throttle]      Nothrottle[0]  Nothrottle[0]
emc0           recoveryoption[errorretry]    Timebound[300] Timebound[300] 
emc0           redundancy                    0              0
emc0           dmp_lun_retry_timeout         0              0
emc0           failovermode                  -              -              
emc0           latency_threshold_difference  200            200

NOTE: Different threshold values can be configured for specific enclosures.

The user can increase or decrease the default latency threshold value for a specified enclosure:

Sample syntax

# vxdmpadm setattr enclosure emc0 latency_threshold_difference​​​​​​​=220

Whilst the DMP "latency_threshold" feature can be disabled for an enclosure. However, Veritas does not recommend disabling it, as this can hide underlying performance issues.

The feature can be disabled, by defining  latency_threshold_difference​​​​​​​ set to 0 for a specific enclosure:

Sample syntax

# vxdmpadm setattr enclosure emc0 latency_threshold_difference​​​​​​​=0


Please contact Veritas Technical Support if you require further guidance.