How to find out if a file is stored in file system caches,

Question

Docs for NSDataReadingOptions state:

NSDataReadingUncached

Hint indicating that the file should not be stored in the file system caches. To read data once and discard this option can improve performance.

It all makes sense. I am curious if there is a way to find out if the file is in the sysem file caches.

For example, if I need to do a large amount of file reading, it might make sense to prioritize reading files that are already in the cache.

Answer 1

I am afraid that we can only allow some assumptions here, since there is no official documentation on this issue.

• The iOS file system must be a hierarchical file system (HFS), the same as for OS-X (see iOS HFS file system? )
• HFS uses Unified Buffer Cache (UBC)
• UBC caches file data in chunks, not the whole file - this is the first point, even if part of the file is cached, you cannot know if the entire file is cached
• There are no APIs or kernel commands for managing UBC content (so this answers your question with NO).

Some interesting reading links:

• UBC testing
• This guy was trying to get some information about UBC content under (jailbreak) iPhone
• The only control over the file system cache specified in the documentation is the same flag that you also found.

You have the ability to quickly find file access to map a file to a virtual memory page, as described in the same Apple document.