Checking replication performance using vxrlink stats
The vxrlink stats command reports detailed information about replication statistics, which can be used to assess network problems. This information about the network performance can be used to determine the optimum network configuration for efficient use of system resources. The vxrlink stats command can be executed only from the Primary. The Monitor view option from the VEA is the parallel for the vxrlink stats command output in the GUI and is available both from the Primary and Secondary.
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All the statistics that the vxrlink stats command displays, are reinitialized when the replication is restarted, either because of a user command or because of the network or server outage. |
The following table describes the output of the vxrlink stats command.
Table: vxrlink stats command output: Information Messages
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Field Name
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Description |
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#
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Displays the number of messages transmitted.
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Blocks
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Displays the number of blocks that are transmitted to the Secondary RVG. One block consists of 512 bytes.
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RT (msec)
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Displays the average round-trip time.
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Delays
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Displays the delay that Volume Replicator introduces while sending the packets, if it is flow controlled. Usually, delays are introduced when there are errors on the link or the outstanding bytes for flow control have been exceeded for a single message.
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The following table describes the output of the vxrlink stats command.
Table: vxrlink stats command output: Error Information
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Field Name
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Description |
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Timeout
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Displays the number of time-out errors. A time-out error occurs when an acknowledgement for a message is not received from the remote host within the computed time-out period. The time-out period is automatically adjusted for optimum performance based on round-trip time (RT).
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Stream
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Displays the errors that occur while sending the updates on the network, which can include errors due to insufficient memory, errors returned by the underlying protocol driver and so on.
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Memory
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Displays the number of memory errors. Memory errors generally occur when the Secondary is unable to store the out of order packets that it receives. One reason for this may be because the Secondary has insufficient buffer space to handle incoming messages or the earlier messages still have some packets pending. This can be fixed by increasing the NMCOM_POOL_SIZE tunable on the Secondary.
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The following table describes how the flow control reacts to the errors that are displayed for vxrlink stats command.
Table: vxrlink stats command output: Flow control
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Field Name
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Description |
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NW Bytes
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Displays the number of bytes that can be transmitted without flow controlling and introducing any intervening delays.
If an RLINK does not experience network errors, Volume Replicator steadily increases the NW Bytes to permit more data to be transmitted. If an RLINK experiences network error, Volume Replicator tries to perform flow control by reducing this number. The minimum value is 5000 bytes.
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NW Delays
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Displays the delay that Volume Replicator may introduce while sending the packets, if it was flow controlled. Usually, delays are introduced when there are errors on the link or the outstanding bytes for flow control have been exceeded for a single message.
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Timeout
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Displays the current time-out value in milliseconds. This value is computed dynamically. If an acknowledgement for a message is not received from the remote host within this value, the message is considered lost and is retransmitted.
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