Hit ratio dropped significantly after recent upgrades

[Justin Lloyd]

I actually have the TTL set to 48h now and it seems to work well, but I'm not sure if there's anything specific I should do regarding invalidation. In general, wikis don't allow for deleting or even changing images; you just upload new ones and the wiki software keeps previous versions in the revision history. However, I am concerned about invalidating cached images when new versions are uploaded so that the previous version is purged from the cache, but I'm not sure about the most appropriate way to handle that from a Varnish or MediaWiki perspective. If you have any thoughts on that, I'd appreciate it.

-----Original Message-----
From: Dridi Boukelmoune [mailto:dridi at varni.sh] 
Sent: Wednesday, December 14, 2016 7:20 AM
To: Justin Lloyd <justinl at arena.net>
Cc: Florian Tham <fgtham at gmail.com>; varnish-misc at varnish-cache.org
Subject: Re: Hit ratio dropped significantly after recent upgrades

On Wed, Dec 14, 2016 at 2:58 PM, Justin Lloyd <justinl at arena.net> wrote:
> So after increasing the TTL for thumbnail images to 4 hours, the hit ratio got to 65-70%, objects in memory got up to around 150k before tapering off (due to lots of expirations starting after 4 hours, as to be expected) and slowly dipping back down to around 100k before starting on the upswing again. I'm continuing to test increasing the TTL, setting it to 24h and we'll see if we have any problems reported as a result in case that's too long, but at any rate we definitely appear to have found the smoking gun and I know where to tinker to try to better optimize things.
>
> I'm not sure why this changed with the upgrades, whether it was something in MediaWiki or Varnish, but at least I know where to spend cycles on optimizations.
>
> Thank you all very much for the help!

Hi,

Thanks for the feedback and glad to see things back to normal.
Consider sharing your findings with the MediaWiki folks as they will likely know better how to deal with the thumbnails.

If images/thumbnails aren't changing often, you can safely increase the TTL (ideally directly from MediaWiki) as long as you have an invalidation strategy in place.

Cheers