[RFE] configuration option to tune oplog size

[Felipe Reyes]

Sometimes users don't enable-ha at the beginning of their journey with juju, when they decide to do it is possible that the database has gotten too big (logs, many models, images, etc) and mongodb could get stuck when replicating the data to the new node, other situation in which a small oplog size could affect a juju controller is when one of the replicas is powered off for maintenance.

Now that the leadership logic is being moved out of mongodb the writes are less, without the update-status changes in the database the writes are even less, but still a busy environment with DEBUG level set for the units and controller(s) may produce a considerable amount of writes.

References:

https://docs.mongodb.com/manual/core/replica-set-oplog/
https://docs.mongodb.com/manual/tutorial/change-oplog-size/
https://jira.mongodb.org/browse/SERVER-26360

[John A Meinel (jameinel)]

We do already have agent.conf "MONGO_OPLOG_SIZE", which I thought we
exposed in controller config. IIRC, we don't let you change it after
bootstrap, as there are some sort of issue around it with mongo (I think
the issue is that you can't change it without restarting mongo, I don't
remember all the details).

But we do have a flag for agent.conf, I don't quite see a way that it is
exposed to the user.

On Mon, Sep 10, 2018 at 4:13 PM, Richard Harding <<email address hidden>
> wrote:

> ** Changed in: juju
> Status: New => Triaged
>
> ** Changed in: juju
> Importance: Undecided => Medium
>
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> [RFE] configuration option to tune oplog size
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[Felipe Reyes (freyes)]

Effectively when using mongodb 3.2 you need to restart the service (see https://docs.mongodb.com/v3.2/tutorial/change-oplog-size/ ), in 3.6 it's no longer needed (see https://docs.mongodb.com/v3.6/tutorial/change-oplog-size/ ), so juju would need to check which version of mongodb is running and if it's >=3.6 accept the change, otherwise return an error indicating why it's not possible to change it

Methods to change the oplog size would be:

juju bootstrap --config mongodb-oplog-size=1234 # mongodb >= 3.2
juju model-config -m controller mongodb-oplog-size=1234 # mongodb >= 3.6

[Canonical Juju QA Bot (juju-qa-bot)]

This bug has not been updated in 2 years, so we're marking it Low importance. If you believe this is incorrect, please update the importance.