Reading between the lines, it sounds as if they're using mmap. There is no "append" operation on a memory mapping, so the file would need to be preallocated before mapping it.

If the preallocation is done using fallocate or just writing zeros, then by default it's backed by blocks on disk, and readahead must hit the disk since there is data there. On the other hand, preallocating with fallocate using FALLOC_FL_ZERO_RANGE or (often) with ftruncate() will just update the logical file length, and even if readahead is triggered it won't actually hit the disk.