Question

Is there a method of quickly determining whether a (4KB-16MB) chunk read from a file is all zeros?

You can iterate over the chunk, checking each byte. There are obvious optimisations, but it remains O(N).

My use case is for sparse files. I would be perfectly happy for a partial solution, such that if the chunk I’ve just read is not backed by any disk storage (i.e. it is a hole) then return true.

Any hints?

Answer 1

This depends on the operating system and sometimes the filesystem. Linux since 2.6.28 has implemented the FIEMAP ioctl(), and ZFS on Solaris implements SEEK_HOLE and SEEK_DATA in lseek().