How does InnoDB access disk?
Default InnoDB page size is 16K. Pages go into a larger 1MB data structure called an extent. MySQL allocates 4 extents at a time.

All of these are stored in one large file, so boundary issues have minimal effect on InnoDB.

You can read more about MySQL file space management.

Modern hard drives have read caches, and they read ahead, so contiguous sectors are already available when you ask for them without having to perform another seek.

Given enough working space (innodb_buffer_pool_size), InnoDB will store much of your indexes and data in memory, only waiting on disk for update transactions and commits.

According to MySQL documentation, it seems MySQL reads an extent (1MB) at a time from disk to the buffer pool, where it will access it a page at a time.