how wsrep_flow_control_paused_ns works?

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jasmine

• November 28, 2018 11:01

Hello!

based on galeracluster.com , wsrep_flow_control_paused_ns provides the amount of time replication was paused in nanoseconds (for me it's replication delay).

So, here are some questions about variable called wsrep_flow_control_paused_ns.

1. After inserting some rows, do we need to type flush table; before typing wsrep_flow_control_paused_ns in order to check the value of it?

2. I do a simple test. I've inserted 300 data rows for 5 times. Every time i insert 300 data, i check the value of wsrep_flow_control_paused_ns. Below are the time in nanoseconds. I dont understand why the replication pause time in some test didn't change.

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  Krzysztof Ksiazek

  • November 28, 2018 11:08

  Hi,

  First of all, wsrep_flow_control_paused_ns is not a replication delay. It tells you for how long time the replication has to be stopped due to some nodes having troubles in catching up. You may have zero flow control but replication delay will still be there. Typically, for small transactions, it's not noticeable but it does exist.

  There's no need for flushing tables. It has no effect over the wsrep_flow_control_paused_ns. Why it behaved like in your tests, it is not possible to tell without knowing the full context and how all the metrics changed. Such spike might have been triggered by some external activity. Filesystem cache flush, InnoDB redo log flush, some network performance problems. Basically, anything. You need to do a full investigation in a system which you have instrumentation for at least majority of MySQL and host level metrics to have a hope of understanding why it behaved like that.

  Thanks,
  Krzysztof

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  jasmine

  • November 28, 2018 11:28

  Hi!

  Thankyou for your answer

  So, i just do a simple test again by inserting 1500 data rows for 5 times. Here is the time in nanosecond.

  

  I noticed that the time is getting higher. Is it possible to get the lower time after inserting data several times?

   

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  Krzysztof Ksiazek

  • November 28, 2018 11:35

  Hi,

  MySQL treats its metrics as either counters or gauges (at least I don't remember any other option from the top of my head). Gauge (such as, for example, Threads_connected) shows the state of the metric at the given time. Counter, on the other hand (just like wsrep_flow_control_paused_ns is) is always increasing. To tell how it changed, you have to calculate the delta between previous measurement and the last one. This will tell you how the counter changed in the meantime. I hope this helps.

  Thanks,
  Krzysztof

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  jasmine

  • November 28, 2018 12:24

  Thankyou so much Krzysztof, it's really helpful :)

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