For an image hosting web application:
For my stored images, is it feasible to create thumbnails on the fly using PHP (or whatever), or should I save 1 or more different sized thumbnails to disk and just load those? Please explain your reasoning.
Any help is appreciated, thanks.
It would be much better to cache the thumbnails. Generating them on the fly would be very taxing on the system.
Save thumbnails to disk. Image processing takes a lot of resources and, depending on the size of the image, might exceed the default allowed memory limit for php. It is less of a concern if you have your own server with only your application running but it still takes a lot of cpu power and memory to resize images. If you're considering creating thumbnails on the fly anyway, you don't have to change much - upon the first request, create the thumbnail from the source file, save it to disk and upon subsequent requests just read it off the disk.
It depends on the usage pattern of the site, but, basically, how many times do you expect each image to be viewed?
In the case of thumbnails, they're most likely to be around for quite a while (the image is uploaded once and never changed, so the thumbnail doesn't change either), so it's generally worthwhile to generate when the full image is uploaded and store them for later. Unless the site is completely dead, they'll be viewed many (hundreds or thousands of) times over their lifetime and disk is a lot cheaper than latency these days. This also becomes more significant as load on the server increases, of course.
Conversely, for something like stock charts that get updated every hour (if not more frequently), that would be a situation where you'd do better to create them on the fly, so as to avoid wasting CPU time on constantly generating images which no user will ever see.
Or, if you want to get fancy, you can optimize to handle either access pattern by generating the images on the fly the first time they're needed and then showing the pre-generated one afterwards, up until the data it's generated from changes, at which point you delete it so that it will be regenerated the next time it's needed. But that would be overkill for something as static as thumbnails, IMO.
I use phpThumb, as it's the best of both worlds. You can create thumbnails on the fly, but it automatically caches the images to speed up future requests. It creates a nice wrapper around the GD and ImageMagick libraries. Worth a look!