Why is /var/cache/nginx a volume?

[dpwspoon]

I would like to know the reasoning on why /var/cache/nginx is a volume.

As an administrator of docker-nginx is this a folder I should archive by default?

[sherter]

I guess this is just for better performance. By declaring an explicit volume you bypass docker's copy-on-write backend (e.g. aufs) and bind-mount the data directly from the host filesystem.
Images like mysql do this for the same reason.

[thresheek]

It is there so you could re-use caches (if any) between nginx upgrades.

[dpwspoon]

I discovered this when I was trying to learn docker and I noticed I was unintentionally orphaning docker volumes.

It seems to me that there are several files you may want to keep between updates such as the conf directories. I've found this issue on why for nginx most of those are not included in the volume.

Other then the potential performance reasons mentioned by @sherter, I can't see why the cache would be a special case. As such perhaps it may be better to remove it for consistency, or have it documented on docker hub/_/nginx readme so people don't make the mistake of orphaning volumes.

[booyaa]

isn't the cache used for static assets?

[Siilwyn]

Other then the potential performance reasons mentioned by @sherter, I can't see why the cache would be a special case. As such perhaps it may be better to remove it for consistency, or have it documented on docker hub/_/nginx readme so people don't make the mistake of orphaning volumes.

I agree with this, at least document it in the readme.

[dpwspoon]

updated the dockerfile with the reasoning provided by @sherter via pull request

[Siilwyn]

👍 Nice!