If you store images on your Nextcloud there is a big change that you have previews enabled. Previews are used for the tiny thumbnails in the file list but also for scaled down images in gallery for example. Because nobody wants transfer their 30 mega pixel photos all the time.

In Nextcloud 11 we have several nice improvements for you regarding previews. Including an shiny new app to pre-generate previews!

Serving previews

First of all we changed the way previews are served to the end user. If a preview was generated we would first construct an image object in memory and then serve the data from that image to the user. This created a lot of overhead.

So the first step is skipping the image parsing in memory. If a preview is created once we just serve the data from that file. This saves precious RAM and CPU cycles.

Etag validation

By default we cache previews for 24 hours. However, after those 24 hours we would just request the same preview object again. While for a majority of the preview files nothing would have changed at all.

We now actually look at the headers a client sends us and if it is the same file (etag/last modified date). We just return the good old 304 status code. To indicate nothing has changed. We can do this by just accessing the database. So no need to go to the slow storage unless we have to actually serve preview data.

Limiting preview sizes

Let say you request a preview of an image of 250 by 250. But the next time you request an image of 252 by 252. Now it makes no sense to generate a new preview. We should serve similar preview sizes as the same file. There was already some code that handled this but we upgraded it in Nextcloud 11.

The algorithm we use now rounds your image up to the nearest power of 2. So you get at least an image of the size you requested but it might be a bit bigger. But your browser is also very good at resizing it a bit for you then. This makes sure that your disk does not fill up with 10K previews per file and that the server can often just serve the same file.

Shared previews

Up until now we would generate previews per user. So if Alice shared a folder full of their holiday pictures with Bob we could generate previews for both Alice and Bob. This separation lead to multiple issues:

• Double the space required
• Double the CPU cycles for preview generation
• If Bob modified a file the preview of Alice would not get updated
• When Bob opens the folder all the previews still have to be generated which requires him to wait until he can browse them

As of Nextcloud 11 we share previews. Which means that we have a single location to store previews. This solves the problems Alice and Bob had as described above.

Pre-generation

The downside of the new preview approach is that we need to drop all the existing previews. Because of certain bugs in the old preview system just moving stuff over is not possible.

To over come this we have been working on an app. This app allows you to do a one time run to generate all the previews.

However the app is capable of doing more. It will listen to writes and keep a list of all those files. Then once you run a command it will start requesting previews for all those files. This can make your overall experience more smooth.

That way you can lets your server generate previews during the night for example.

Get the app in the appstore.