This is visible on the “ InnoDB Buffer Pool Data” graph: dirty pages are colored in yellow. What is interesting here is that the number of dirty pages went down to zero. InnoDB Buffer Pool activity is also stopped. You may see this in the “ InnoDB Logging Performance” graph: InnoDB still uses log files but only for background operations. This is unusual even on a server that does not handle any user connection: InnoDB always performs background operations and is never completely idle. More importantly, we see that all I/O operations were stopped. This means that something prevents InnoDB from performing its operations. Therefore the next step for figuring out what is going on would be to examine graphs in the “ MySQL InnoDB Details” dashboard.įirst, we are seeing that the number of rows that InnoDB reads per second went down to zero, as well as the number of rows written. I, like most MySQL users, used InnoDB for this example. But to understand the picture better let’s examine storage engine graphs. This by itself shows that something unusual happened. At the same time the number of “ MySQL Temporary Objects” lowered down to zero. In this picture, you see that the number of active threads is near maximum. The next screenshot was taken when the server was stalling: Or, if the client application processes retrieved data. Let’s review the dashboard “ MySQL Instance Summary” and its graph “ MySQL Client Thread Activity” during normal operation:Īs you see, the number of active threads fluctuates and this is normal for any healthy application: even if all connections request data, MySQL puts some threads into idle states while they need to wait while the storage engine prepares the data for them. In the case of stalls, you will see that either some activity went to 0 or, otherwise, it increased to high numbers. If you look at its graphs and notice that many of them started showing unusual behavior, you need to react. It is always better to know about the stall from a monitoring instrument rather than from your own customers. As a result, the application will slow down and then will stop responding. Nobody wants it but database servers may stop handling connections at some point. I will use only one typical situation for the MySQL server stall in this example, but the same dashboards, graphs, and principles will help you in all other cases. We have other server running with older version of v4.0.13 and somehow we could run its version of mongodump to backup the abovementioned DB instance of v4.2.In this blog, I will demonstrate how to use Percona Monitoring and Management (PMM) to find out the reason why the MySQL server is stalling. We can connect to the db using mongo cli and our app can also make the DB connection successfully. … will listen for SIGTERM, SIGINT, and SIGKILL When we ran the same command with -vvvvv option, we only saw 2 extra lines, ie. In the mongodb log file, we saw many lines of the following error msg before it failed with the above error message:. T22:55:41.944+0800 Failed: can’t create session: could not connect to server: server selection error: server selection timeout, current topology: When we tried to do mongodump, we hit with the following error :. We have 3 servers to run the 3 instances. We are running percona mongodb v4.2.14 with replica.
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