Bug 1436635

Summary: mongodb claims 3.4G disk space in /var
Product: Red Hat OpenStack Reporter: Yolanda Robla <yroblamo>
Component: openstack-tripleoAssignee: James Slagle <jslagle>
Status: CLOSED NOTABUG QA Contact: Arik Chernetsky <achernet>
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Version: 12.0 (Pike)CC: aschultz, jdanjou, mburns, michele, rhel-osp-director-maint, royoung
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Last Closed: 2017-06-22 09:25:15 UTC Type: Bug
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Description Yolanda Robla 2017-03-28 11:06:04 UTC 
Description of problem:

When installing mongodb on a fixed size partition, it raises the need of having 3.4G reserved on /var space. This causes the base image to grow, and after some deployment it shows not being used. It is required for mongo journaling, and it can be avoided using the --small-files flag on mongo.

Actual results:

Mongo fails to deploy when /var space is lower than 3.4G claiming this space for journaling.


Expected results:

No hard disk requirement shall be needed for that, using the --small-files flag


Additional info:

We'd like to add this config to TripleO, but we are unsure of the pros/cons of it. We need advice on that, understand the advantages of not having --small-files, and the possible problems if we enable it.
Comment 1 Michele Baldessari 2017-03-29 20:13:31 UTC 
I am no mongo expert, so take this with a pinch of salt.

The option we talk about seems to be described here:
https://docs.mongodb.com/manual/reference/configuration-options/#storage.mmapv1.smallFiles
"""
When true, MongoDB uses a smaller default file size. The storage.mmapv1.smallFiles option reduces the initial size for data files and limits the maximum size to 512 megabytes. storage.mmapv1.smallFiles also reduces the size of each journal file from 1 gigabyte to 128 megabytes. Use storage.mmapv1.smallFiles if you have a large number of databases that each holds a small quantity of data.

The storage.mmapv1.smallFiles option can lead the mongod instance to create a large number of files, which can affect performance for larger databases.

The storage.mmapv1.smallFiles setting is available only for mongod.
"""

So it seems to make sense only if we use many small databases as opposed to a few larger ones. Since we only have a very few dbs:
tripleo:PRIMARY> show databases
admin        (empty)
ceilometer   0.078GB
local       48.055GB

The tradeoffs for this options seem to be not in favour of performance here
Comment 2 Yolanda Robla 2017-03-30 08:09:26 UTC 
So we better allocate enough disk space for the large journal when building the image?
Comment 3 Michele Baldessari 2017-03-30 08:34:20 UTC 
That would be my take yes. Although I'd like we take a bit of higher-level approach on this and first do an estimation of how much data we need in /var in general. I mean we also store galera data there when galera and mongo exist on the same role, so a small /var would affect an environment quite a bit, no? Can we maybe take a step back and outline the reasons for your efforts here just to clarify the context a bit more?
Comment 4 Yolanda Robla 2017-03-30 08:37:59 UTC 
That's the blueprint i'm working on , and that explains the needs for different volumes:

https://blueprints.launchpad.net/tripleo/+spec/build-whole-disk-images