Using Percona Memory Engine¶

As of version 3.2, Percona Server for MongoDB runs with WiredTiger by default. You can select a storage engine using the --storageEngine command-line option when you start mongod. Alternatively, you can set the storage.engine variable in the configuration file (by default, /etc/mongod.conf):

Data created by one storage engine is not compatible with other storage engines, because each one has its own data model. When changing the storage engine, you have to do one of the following:

• If you simply want to temporarily test Percona Memory Engine, change to a different data directory with the --dbpath command-line option:

  $ service mongod stop
  $ mongod --storageEngine inMemory --dbpath <newDataDir>
  

  Note

  Make sure that the user running mongod has read and write permissons for the new data directory.

  Note

  Although Percona Memory Engine stores all data in memory, some diagnostics and statistics are still written to disk. This is controlled using the --inMemoryStatisticsLogDelaySecs option.

• If you want to permanently switch to Percona Memory Engine and do not have any valuable data in your database, clean out the default data directory and edit the configuration file:

  $ service mongod stop
  $ rm -rf /var/lib/mongodb/*
  $ sed -i '/engine: \*inMemory/s/#//g' /etc/mongod.conf
  $ service mongod start
  

• If there is data that you want to migrate and make compatible with Percona Memory Engine, use the mongodump and mongorestore utilities:

  $ mongodump --out <dumpDir>
  $ service mongod stop
  $ rm -rf /var/lib/mongodb/*
  $ sed -i '/engine: \*inMemory/s/#//g' /etc/mongod.conf
  $ service mongod start
  $ mongorestore <dumpDir>
  

Configuring Percona Memory Engine¶

You can configure the Percona Memory Engine using either command-line options or corresponding parameters in the /etc/mongod.conf file. The configuration file is formatted in YAML. For example:

storage:
  engine: inMemory
  inMemory:
    engineConfig:
      inMemorySizeGB: 140
      statisticsLogDelaySecs: 0

Setting parameters in the previous example configuration file is the same as starting the mongod daemon with the following options:

mongod --storageEngine=inMemory \
  --inMemorySizeGB=140 \
  --inMemoryStatisticsLogDelaySecs=0

The following options are available (with corresponding YAML configuration file parameters):

option --inMemorySizeGB¶

Config:	storage.inMemory.engineConfig.inMemorySizeGB
Default:	50% of total memory minus 1024 MB, but not less than 256 MB

Specifies the maximum memory in gigabytes to use for data.

option --inMemoryStatisticsLogDelaySecs¶

Config:	storage.inMemory.engineConfig.statisticsLogDelaySecs
Default:	0

Specifies the number of seconds between writes to statistics log. If 0 is specified then statistics are not logged.