50 Years in Filesystems: 1974 Unix V7

https://blog.koehntopp.info/2023/05/05/50-years-in-filesystems-1974.html

by fh973 • 3 years ago

98 14 3 years ago

blog.koehntopp.info

senectus1• 3 years ago

oi! steady on.. I was born in 1974 and I'm not 50 YET!

iohead• 3 years ago

Well, V7 UNIX is rather arbitrary as a reference point for comparison. There was what we now call as V1 UNIX, which would be circa 1971/72, so that's certainly 50+ years old. Compared to what's shown in the article, some of the data structure fields and constants would be different and/or missing in the still earlier versions. DIRSIZ would be 8, for example.

musicale3• 3 years ago

Well the author refers to Bach's book (though I thought that was System V?) Another option would be V6 with Lions' commentary.

musicale3• 3 years ago

There can be only one writer active per file, which kills concurrency

Multiple writers contending over a single resource is what kills concurrency. Single writer is a good design.

It's also helpful for distributed implementations.

monocasa4• 3 years ago

A file is arguably N resources, where N is the number of granular elements of your file. Probably pages, but could be blocks or bytes too depending on where in storage it's canonical copy lives.

musicale3• 3 years ago

Point taken. In any case, single-writer helps with concurrency and consistency. For V7 Unix, I think one solution could be to split the file into blocks, each with a single writer. Admittedly this makes reads of the whole file more costly, so one has to be careful about the block size.

tremon• 3 years ago

But those N resources are not independent, which is why it makes sense to think of them as one singular resource. For example, if you were to remove the first byte from the file, or prepend a byte, all the other bytes' addresses would change. And arguably, we already have an interface to logically group different filesystem objects together yet retain the ability to address them individually: it's called a directory.

I'm sure you could define a more granular format to address all elements within a file individually (for example, a lot of files in a typical Unix /etc directory have rows and fields), but people would call that interface an object store rather than a filesystem.

ElectricalUnion• 3 years ago

But those interfaces [to allocate bytes/pages before other bytes/pages, allocate bytes/pages in the middle of bytes/pages, to prune bytes/pages before other bytes/pages] to manipulate said parts are non-portable between *NIX implementations, usually limited to certain filesystems with specific implementation-specific minimum byte sizes.

So like how the OS pretends filesystems are in a tape, the filesystem pretends that the files are in a tape.

And tape splicer is optional with a not-standard interface (logical volume management, fallocate, etc).

aap_• 3 years ago

A bit strange to talk about 1974 but look at code from v7 (1979).

rahen• 3 years ago

Indeed. 1974 was the release year of V6, not V7.

50 Years in Filesystems: 1974 Unix V7

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