in reply to Loading only portions of a file at any given time

What TKCooper said, but ... you could also do something more complex like build an "index" and store it in another file and then use that index to determine which parts to load when. But, the cost of that method (in time and complexity and developing the thing that reads based on index and in maintenance) may out-weigh the benefit you gain. Though, if this is a commonish thing to want, maybe everyone who uses FASTA could benefit from a module that does this indexing? I dunno.