hdmcw-files.qmd

# Episode 12: Files (WIP) {.unnumbered}

Files have been the key metaphor for computer data since, like, the 1950s.  Conceptually they seem
simple: a collection of bits stored on some semi-permanent device with some associated metadata that identifies
that collection of bits as an isolated thing with a name.  But that description is simplistic.  Files
are deep eldritch magic.

First, there are the details of that semi-permanent storage device.  Over my years in the biz that has been
everything from paper tape to the NAND Flash device (devices?) that make up my computer's 1 TB SSD.  They
are usually read/write although i've had read-only devices.  There has to be some low-level way for code to
read and write bits to specified places on those devices.  Typically there's a piece of controller hardware
that facilitates the process of taking bits from memory and moving them to storage.  I _think_ that in my
computer this is an NMVE controller, based on this from the System Report:

![NVMExpress System Report](nvme.png)

I confess this confuses me a bit, because i was under the impression that NVME is designed for PCIe bus devices,
whereas the "Physical Connection" listed for the NVME controller is "Apple Fabric".

SSDs and hard drives are _block devices_, meaning that reading and writing are done in chunks (blocks) of some specified size.

You can't talk about files without talking about the _file system_, which for my computer is the 
Apple File System (APFS).  Informally, we sometimes think of the file system as the way that directories 
are organized, but it's much more 
(the [Apple File System Reference](https://developer.apple.com/support/downloads/Apple-File-System-Reference.pdf)
document is 181 pages long).  As with most things related to Macs, the best thing written about APFS is on
The Eclectic Light Company:

[APFS: Files and clones](https://eclecticlight.co/2024/03/20/apfs-files-and-clones/)

[APFS: Directories and Names](https://eclecticlight.co/2024/03/25/apfs-directories-and-names/)

[APFS: Containers and Volumes](https://eclecticlight.co/2024/04/02/apfs-containers-and-volumes/)

Why do we need this elaborate system to manage files?   The idea of a file as a metaphor might make you
think (if you are of a certain age) as a set of related documents, all tucked into a _folder_, which is then
placed in a file cabinet in some sequential order, maybe alphabetical.  New documents belonging in that folder
are added to it and the folder goes back in the cabinet in it's proper place.

Computer files don't work that way.  It's true that bytes are written