How to know if a file is "stored in the file-system caches"

Question

The docs for NSDataReadingOptions state:

NSDataReadingUncached

A hint indicating the file should not be stored in the file-system caches. For data being read once and discarded, this option can improve performance.

That all makes sense. I'm curious if there's a way to know if a file already resided in the file-sysem caches.

For example, if I need to perform a large amount of file reading, it might make sense to prioritise reading files which already reside in the cache.

Answer 1

I am afraid here we can relay only some suppositions as no official documentation is available on this.

• The file system of iOS is supposed to be Hierarchical File System (HFS) the same as that one of OS-X (see iOS filesystem HFS?)
• HFS uses Unified Buffer Cache (UBC)
• UBC caches file data in chunks rather then the whole file - this is the first point, even if part of the file is cached you can't know whether the whole file is cached
• There are no any APIs or kernel commands to control the contents of UBC (so this answers your question with NO).

Some interesting links to read:

• Testing the UBC
• This guy tried to get some info about the contents of UBC under (jailbreaked) iPhone
• The only control over the file system cache mentioned in the documentation is the same flag you also found.

Your option to ensure that a file can be accessed quickly is to map the file onto a page of the virtual memory as described in the same doc of Apple.