Thumbnail storage strategy

Question

I am working on a portion of an app that requires a "Photos" type presentation of multiple thumbnail images. The full size images are quite large, and generating the thumbnails every time is taking too long, so I am going to cache the thumbnails.

I am having a hard time determining how best to store the thumbnails on the filesystem once I create them. I can think of a few possibilities but I don't like any of them:

• Save the thumbnail in the same directory as the original file, with _Thumb added to the filename (image.png and image_Thumb.png). This makes for a messy directory and I would think performance would become a problem because of reading so many different files to load at once.
• Save the thumbnails in their own sub-directory, with the same filename as the original. I think that this is slightly cleaner, but I'm still opening lots of different files.
• Save all of the thumbnails to a Thumbnails file. I think that this is commonly done in Windows and OS X? I like the idea because I can open one file and read multiple thumbnails from it, but I'm not sure how to store all of them in the same file and associate them with the original files. EDIT: I thought of using NSKeyedArchiver/unArchiver but from what I can find, anytime a thumbnail is added/removed, I would have to re-create the entire archive. Perhaps there is something that I am overlooking?
• EDIT Store the thumbnails in a core data/sqlite database file. I have heard over the years that it is a bad idea to store images in a database file due to slow performance and the possibility of database corruption on writes that take a (relatively) long time to complete. Does anyone have experience using either one this way?

Any suggestions on the best approach to take?

Answer 1

I would go for the second option. On iDevices you use flash memory. Performance penalty for accessing many files is very low comparing to HDDs. Also you can cache some in memory to prevent reading one and the same file too often. SDWebImage caching mechanism contains a great sample how to do it.

The third option - using one file for that would probably mean using database file. You could have some performance improvements there if you store uncompressed data. You'll need to do some performance tests because loading more data (uncompressed form of the thumbs), might slow it down saving CPU for more storage access.

Combined approach would be to store thumbnails as files but in uncompressed format (not .jpg, .png etc.)

A fourth option worth considering, as long as the thumbnails are reasonably small: save them in CoreData.