Writing a file system from scratch in Rust (opens in new tab)

A very accessible (though dated) intro to filesystems is Practical File System Design, by Dominic Giampaolo.

PDF link: http://www.nobius.org/practical-file-system-design.pdf

And for code there is TFS https://github.com/redox-os/tfs

Soooo... how does it work?

I'm not asking about the structure or how it's organized. I mean... is the filesystem in a file or... how?

Background: I mostly do embedded stuff so at a glance I would have expected low level primitives (like, HW interactions, registers and stuff) but I see none. So maybe, my expectation, when tacking a problem, of interacting with the HW directly, does not stand in modern environments.

Even better, but unrelated question... how the heck does a x86 OS request data from the HDD?

This is really cool, I wish someone would fund it.

> It would be nice if the intro had a brief explanation of why a disk needs to be divided into blocks.

One reason is that HDDs simply don't have a byte-wise resolution, so there's little point talking to HDDs in sub-sector units. Sectors are usually 512 bytes to 4k.

A second reason is being able to simply address the drive. Using 32b indices, if you index bytewise you're limited to 4GB which was available in the early 90s. With 512 bytes blocks, you get an addressing capacity of 2TB, and with 4k blocks (the AF format), you get 16TB. In fact I remember 'round the late 90s / early aught we'd reformat our drives using higher-size blocks because the base couldn't see the entire thing.

A drive (be it HDD or SSD) is a block device, all drives are divided into blocks/sectors since they are a rather lossy medium. Data written to a bit may not necessarily be read accurately and so the solution is to have an Error-Correcting-Code that will allow to recover from some number of badly read bits (above that and you get a medium error from the drive). Since an ECC for a byte is a bit excessive the drives use blocks of 512 bytes or 4096 bytes, one reason for the move from 512 to 4096 is that the ECC becomes more efficient.

HDDs also have small "gaps" that have headers to locate the sectors (in the distant past you could do a low-level format to correct these gaps as well).

SSDs do not have these gaps but they do need the ECC.

In the bad old days we stored data on spinning-rust harddrives. In order to find the data you wanted you had to position the reader head. This took several milliseconds. You could read a lot of data in that time. So basically if you took the trouble to seek somewhere you better read a bunch of data to make it worthwhile.

People would even run defragmenters, that would rearrange file blocks so they were stored continously and the whole file could be read without seeeking.

The disk / inodes need to know where to start looking for a file's contents, like the address in memory for RAM. Or like the mail: We subdivide by city, then ZIP, then street, then address.

So the inode says "The data for my file starts at block 72 and is 3 blocks long" (or something like that). The disk then goes there, and reads blocks 72,73,74.

Each block is 4KiB large often, so if you have a 10KiB file, you still take up ceiling(file size/block size) blocks.

That's why there is a difference between "File size" and "Size on disk" when you look at disk usage summaries.

Yes. Oracle has done this (ASM) to eliminate overhead, implement fault tolerance and provide a storage management interface based on SQL, for example.

I once made a 'file system' to mount cpio archives (read-only) in an embedded system. Cpio is an extremely simple format to generate and edit (in code) and mounting it directly was very effective.

You may be interested in a paper written by the Ceph team: "File Systems Unfit as Distributed Storage Backends: Lessons from 10 Years of Ceph Evolution"

https://www.pdl.cmu.edu/PDL-FTP/Storage/ceph-exp-sosp19.pdf

There are definitely some significant benefits you can get from managing your own storage, rather than using a filesystem.

Yes, this is common in database engines. Doing so allows you to optimize the file system along a very different set of performance tradeoffs and assumptions than a typical generic file system. Beyond that, it also gives you direct control of file system behavior, the lack of which is a source of code complexity and edge cases. This is not transparent to the database, something like PostgreSQL would need to have its storage layer redesigned to explicitly take advantage of the guarantees.

It isn't just about performance gains, which are substantial, it also greatly simplifies the design and code by eliminating edge cases, undesirable behaviors, and variability in behavior across different deployment environments.

Any more specifics?

> How soon until someone builds an Operating System developed in Rust?

Redox[1] has been around for almost as long as Rust has. I first heard about it 4-5 years ago.

They had an interesting competition a while back challenging people to figure out how to crash it.

1. https://www.redox-os.org/