Messages in this thread | | | Date | Thu, 31 Oct 2002 05:12:40 +0000 | From | Phillip Lougher <> | Subject | Re: ANNOUNCEMENT: Squashfs released (a highly compressed filesystem) |
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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
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