Putting a Rails test database on Ramdisk

Updated: 26 Jan 2014

Want to run your Rails tests in a super fast Ramdisk?

Ingredients

  1. Postgres (8.0 or later)
  2. Ubuntu (12.04 is what I tested but will likely work on just about any version)

Method

My test database is only about 30MB when seeded, so I can get away with a 200MB Ramdisk. Here's a quick bash script to create a 200MB Ramdisk and a tablespace on that disk:

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echo "Creating ramdb, make sure your test db has 'tablespace test'"
echo "in database.yml"
sudo mkdir -p /mnt/ram/pgtest
sudo mount -t ramfs -o size=200m ramfs /mnt/ram/pgtest
sudo chown postgres:postgres /mnt/ram/pgtest

psql postgres -c \
"create tablespace fastspace location '/mnt/ram/pgtest'"

Once you have done that, edit your database.yml and get it to be created in the new tablespace:

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test:
  adapter: postgresql
  database: mydatabase_test
  ...
  tablespace: fastspace

A bash script to drop everything again:

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psql postgres -c "drop database mydatabase_test"
psql postgres -c "drop tablespace fastspace"
sudo rm -rf /mnt/ram/pgtest
sudo umount /mnt/ram

My "benchmarks"

Disk

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1.
real  3m43.338s
user  2m35.018s
sys 0m25.614s

2.
real  3m42.426s
user  2m34.654s
sys 0m26.246s

3.
real  3m45.447s
user  2m38.186s
sys 0m26.182s

Ramdisk

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1.
real  3m25.656s
user  2m38.334s
sys 0m25.746s

2.
real  3m23.970s
user  2m36.798s
sys 0m25.682s

3.
real  3m24.451s
user  2m37.226s
sys 0m25.702s

Results

It looks like I saved about 20s. Not that much, considering this is around 1300 tests. Most of my tests seem to be CPU bound.

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