Automating MongoDB Log Rotation

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MongoDB Log RotationIn this blog post, we will look at how to do MongoDB® log rotation in the right—and simplest—way.

Log writing is important for any application to track history. But when the log file size grows larger, it can cause disk space issues. For database servers especially, it may cause performance issues as the database needs to write to a large file continuously. By scheduling regular log rotation, we can avoid such situations proactively and keep the log file size below a predetermined threshold.

MongoDB Log File

In MongoDB, the log is not rotated automatically so we need to rotate it manually. Usually, the log size of the MongoDB server depends on the level of information configured and the slow log configured. By default, commands taking more than 100ms, or whatever the value set for the slowOpThresholdMs parameter, are written into the MongoDB log file. Let’s see how to automate log rotation on Linux based servers.

Rotate MongoDB Log Methods

The following two methods could be used to rotate the log in MongoDB. The first uses the command shown below from within mongo shell:

The alternative is to use SIGUSR1 signal to rotate the logs for a single process in Linux/Unix-based systems:

The behaviour of log rotation in MongoDB differs according to the value of the parameter logRotate which was introduced in version 3.0 (note that this should not be confused with the logRotate command that we’ve seen above). The two values are:

  • rename – renames the log file and creates a new file specified by the logpath parameter to write further logs
  • reopen – closes and reopens the log file following the typical Linux/Unix log rotation behavior. You also need to enable logAppend if you choose reopen. You should use reopen when using the Linux/Unix log rotate utility to avoid log loss.

In versions 2.6 and earlier, the default behavior when issuing the logRotate command is the same as when using rename, i.e. it renames the original log file from mongod.log to mongod.log.xxxx-xx-xxTxx-xx-xx format where x is filled with the rotation date time, and creates a new log file mongod.log to continue to write logs. You can see an example of this below:

Using the above method, we need to compress and move the rotated log file manually. You can of course write a script to automate this. But when using the logrotate=reopen option, the mongod.log is just closed and opened again. In this case, you need to use the command alongside with Linux’s logrotate utility to avoid the loss of log writing in the course of the log rotation operation. We will see more about this in the next section.

Automating MongoDB logRotate using the logrotate utility

I wasn’t a fan of this second method for long time! But MongoDB log rotation seems to work well when using Linux/Unix’s logrotate tool. Now I prefer this approach, since it doesn’t need the complex script writing that’s needed for the first log rotation method described above. Let’s see in detail how to configure log rotation with Linux/Unix’s logrotate utility.

MongoDB 3.x versions

Start MongoDB with the following options:

As mentioned in the section about Linux’s logrotation utility, you need to create a separate config file /etc/logrotate.d/mongod.conf for MongoDB’s log file rotation. Add the content shown below into that config file:

In this config file, we assume that log path is set as /var/log/mongodb/mongod.log in /etc/mongod.conf file, and instruct Linux’s logrotation  utility to do the following:

  • Check the size, and start rotation if the log file is greater than 100M
  • Move the mongod.log file to mongod.log.1
  • Create a new mongod.log file with mongod permissions
  • Compress the files from mongod.log.2 but retain up to mongod.log.10 as per delaycompress and rotate 10
  • MongoDB continues to write to the old file mongod.log.1 (based on Linux’s inode) – remember that now there is no mongod.log file
  • In postrotate, it sends the kill -SIGUSR1 signal to mongod mentioned with the pid file, and thus mongod creates the mongod.log and starts writing to it. So make sure you have the pid file path set to the same as pidFilepath from the /etc/mongod.conf file

Please test the logrotate manually using the created /etc/logrotate.d/mongod.conf file to make sure it is working as expected. Here’s how:

 

Adaptations for MongoDB 2.x and earlier, or when using logRotate=rename

Since the introduction of logRotate parameter in MongoDB 3.0, the log rotate script needs an extra step when you are using logRotate=rename or when using <=2.x versions.

Start the MongoDB with the following options (for 2.4 and earlier, ) :

Start MongoDB with the following options YAML format (YAML config introduced from version 2.6) :

The config file /etc/logrotate.d/mongod.conf for MongoDB’s log file rotation should be set up like this:

In this case, the logrotate utility behaves as follows:

  • Check for the size, and start rotation if the log file size exceeds 100M
  • Move mongod.log file to mongod.log.1
  • Create a new mongod.log file with mongod permissions
  • MongoDB continues to write to the old file, mongod.log.1
  • In postrotate, when the SIGUSR1 signal is sent, mongod rotates the log file. This includes renaming the new mongod.log file (0 bytes) created by logrotate to mongod.log.xxxx-xx-xxTxx-xx-xx format and creating a new mongod.log file to which, now, mongod starts writing the logs.
  • the Linux command find  identifies mongod.log.xxxx-xx-xxTxx-xx-xx file formats that are sized at 0 bytes, and these are removed

If you enjoyed this blog…

You might also benefit from this recorded webinar led by my colleague Tim Vaillancourt MongoDB Backup and Recovery Field Guide or perhaps the Percona Solution Brief Security for MongoDB.

References:
https://jira.mongodb.org/browse/SERVER-11087
https://jira.mongodb.org/browse/SERVER-14053
https://jira.mongodb.org/browse/SERVER-16821
https://docs.mongodb.com/v2.6/reference/configuration-options/
https://docs.mongodb.com/v2.4/reference/configuration-options/
https://docs.mongodb.com/manual/reference/configuration-options/
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Comment (1)

  • Darshan Jrshan Reply

    nicely documented, well explained with commands,Thanks Vinod,

    September 27, 2018 at 1:39 pm

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