Messages in this thread | | | Date | Fri, 14 Jul 2000 09:37:48 -0700 (PDT) | From | (Jim Gettys) | Subject | Re: Do ramdisk exec's map direct to buffer cache? |
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We suspect that a combination of cramfs and jffs will serve handhelds very well... I can say from first hand experience that compressed ramdisks and/or cramfs gets very old, very quickly: we really want a writable file system, with (read) compression.
We have both cramfs and jffs cranking over right now on the iPAQ, but haven't cut over to using either quite yet (but probably will in the next few weeks sometime. We initially tried using cramfs, but had a flash driver bug we didn't know about at the time so did ramdisk to get the iPAQ to Usenix. But the current state is not ideal.
NOTE: we don't want/need general compression of data being written. Most data being written (in terms of volume) is likely to be in already compressed formats (e.g. note taking via audio which will then be compressed before being written). You don't want to pay the joules or performance to compress the data twice. Most of the flash is likely to be executables or shared libraries.
A scheme Keith Packard proposed which would work very well for read only data is somewhat similar to cramfs: just compress on 4K boundaries and store an offset table, and mark the file as compressed when you are done; do the obvious uncompression on the marked files on read. My intuition is that this might be a performance win even on vanilla machines today.
So we argue that in fact full general compression of files automatically behind the application's back will in fact be highly counter productive on handheld devices. The stuff we will write the most of will already be compressed...
Ted says Keith's scheme can be done with a stacking file system: this would get it for all file system types, which strikes me as a win. We'll do it someday, if no one gets to it in the meanwhile (it will probably still be months before we get to that point).
- Jim
-- Jim Gettys Technology and Corporate Development Compaq Computer Corporation jg@pa.dec.com
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