what is the meaning of the --max-flow-ctl

Fan · December 30, 2020, 4:24pm

Can someone explain the meaning of the --max-flow-ctl parameter, do not understand how this percentage is calculated

Fan · December 30, 2020, 4:35pm

My understanding is that A = $ (when spent on flow control (from the start of pt-osc/pt-archiver) / start executing pt-osc/pt-archiver to the current total duration * 100)
Pt-osc/pt-archiver is suspended when A > —max-flow-ctl specifies the value. Over time, when A < —max-flow-ctl specifies the value, continue to execute pt-osc/pt -archiver
I don’t know if it’s right

Fan · December 30, 2020, 4:35pm

Can someone answer my question?

Michael_Coburn · December 30, 2020, 4:35pm

Hi Fan - this setting in pt-archiver ensures that the archiving routine doesn’t overwhelm a PXC cluster by setting a limit to how far replication can lag other members of the cluster.

From the documentation:
https://www.percona.com/doc/percona-toolkit/LATEST/pt-archiver.html#cmdoption-pt-archiver-max-flow-ctl

And background on PXC Flow Control:
https://www.percona.com/blog/2013/05/02/galera-flow-control-in-percona-xtradb-cluster-for-mysql/
What is flow control?

One of the things that people don’t often expect when switching to Galera is existence of a replication feedback mechanism, unlike anything you find in standard async MySQL replication. It is my belief that the lack of understanding of this system, or even that it exists, leads to unnecessary frustration with Galera and cluster “stalls” that are preventable.

This feedback, called flow control, allows any node in the cluster to instruct the group when it needs replication to pause and when it is ready for replication to continue. This prevents any node in the synchronous replication group from getting too far behind the others in applying replication.

This may sound counter-intuitive at first: how would synchronous replication get behind? As I’ve mentioned before, Galera’s replication is synchronous to the point of ensuring transactions are copied to all nodes and global ordering is established, but apply and commit is asynchronous on all but the node the transaction is run on.

I hope this helps!

Fan · December 30, 2020, 4:35pm

First of all, I want to express my gratitude, but this does not answer my question very well…
I want to know the algorithm of this parameter

carlos.salguero · December 30, 2020, 4:35pm

Hi,

The part of the code checking the lag/flow control is here:
https://github.com/percona/percona-toolkit/blob/3.0/bin/pt-archiver#L4384-L4387

Regards

Fan · December 30, 2020, 4:35pm

thanks a lot !