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Daniel Doubrovkine

aka dB., @awscloud, former CTO @artsy, +@vestris, NYC

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MongoDB SERVER-24384 bit me.

RSpec tests in Mongoid/MongoDB projects would take forever to run, spending multiple seconds creating collections or indexes. Typically, projects using MongoDB (I have a list of at least two dozen here) use database_cleaner to drop the entire database before each test and re-create all indexes to mimic a clean production-like environment. This would now take 5-10 seconds per test.

I opened SERVER-24384, which was closed as a duplicate of SERVER-17675. Turns out, WiredTiger uses one file per collection and one file per index, so in this workload the system is very busy creating and removing files, which explains why it’s so slow.

The workaround is to use the mmapv1 storage engine in development. Here’s my /usr/local/etc/mongod.conf.

  destination: file
  path: /usr/local/var/log/mongodb/mongo.log
  logAppend: true
  dbPath: /usr/local/var/mongodb
  engine: mmapv1
     enabled: false

You’ll have to stop MongoDB, then clear all your data in /usr/local/var/mongodb/ before restarting it.

$ brew services stop mongodb
Stopping `mongodb`... (might take a while)
==> Successfully stopped `mongodb` (label: homebrew.mxcl.mongodb)

$ rm -rf /usr/local/var/mongodb/*

$ brew services start mongodb
==> Successfully started `mongodb` (label: homebrew.mxcl.mongodb)