Re: ANNOUNCEMENT: Squashfs released (a highly compressed filesystem)

Phillip Lougher
2002-10-31T05:11:44Z

Matthew Wilcox wrote:
>Larry McVoy wrote:
>> > A r/w compressed filesystem would be darned useful too :)
>> > mmap(2) is, err, hard. Not impossible, it means the file system
has >to
>> support both compressed and uncompressed files, but it's interesting.
>
>Easier than you think, perhaps. Depends how much compression you're
>after, of course, but here's how Acorn did it in RISCiX (a 4.3BSD
>derivative):
>

Strange you should mention this... I used to work at Acorn with
the guy who did this (Mark Taunton). RISCiX was too early to be 4.3, I
think it was 4.1, but may have been 4.2, certainly no later, this was
1989.

>Pages were 32k (an interesting feature of the MMU...), and the
>underlying filesystem was a fairly vanilla BSD FFS (probably 4k blocks
>with 1k fragments; discs were around 50MB). Each page was written at
a >32k boundary, but compressed. So there were holes in the file where
>other files could store their data. Naturally you waste on average
512 >bytes per 32k page, but I think they managed to get 80MB of unix
distro >onto a 50MB disc this way, so it's nothing to be sneezed at.

They only needed to compress the fs on the R140 (an ARM 2 based
machine). The better R260 had a bigger disk, and so didn't need the
compression. Interestingly, because of the slow disk I/O, compression
made the filesystem access faster.

I had an R140, and space was so tight in the filesystem, you had
to delete part of the distribution before you could even compile
a program!

Phillip