Re: [RFC] Advanced XIP File System


What is it?:
- Borrows much from CRAMFS with Linear XIP patches

Apropos borrowing: borrow the compression from Squashfs.

I've heard people say, "XIP is stupid. Why on earth would I use
expensive slow flash instead of cheap fast RAM?"
[...] If I
take libfoo.so which is about 2MiB and throw it in JFFS2, it
compresses to about 1MiB in flash. If I store libfoo.so as an XIP
file (uncompressed) in XIP CRAMFS or AXFS it takes up 2MiB of flash. That is
1MiB extra flash for XIP. But the JFFS2 version would need
2MiB of RAM to store that library when used while the XIP system uses
0MiB. That means XIP uses +1MiB of flash and -2MiB of RAM for
libfoo.so. So for any extra flash used for XIP, it can save twice
that amount of RAM. The end result can be lower cost systems on the
small end. The secret is to choose what to make XIP and what to
compress on flash.

If 2 MB of RAM are cheaper [as you say] than 1 MB of Flash, where's the
advantage when XIP uses more flash?


Jan Engelhardt
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