Re: JFFS2 mount time

[Ferenc Havasi <havasi@inf.u-szeged.hu>]

David Woodhouse wrote:
> It's looking good, but the kernel really needs to be able to write these
> summaries for _itself_ in order to give a real improvement over the long
> term. If the file system has to be read-only we might as well be using
> cramfs, and if the summary becomes obsolete over time we might as well
> not bother in a lot of cases.

Our plan for it:

We would like to store some additional information in jeb struct:
- a type information, where there this type can be INODE_ONLY and 
ANYTHING_OTHER. This information is easy to detect during mount time.
- a predicted summary size (calculated dinamically). It will be used to 
  decide when to generate the summary. Ceratinly only for INODE_ONLY 
erase blocks.

If I am right every node allocation is done by jffs2_reserve_space(). We 
would like to modify it, and introduce a new interface for it called 
jffs2_reserve_space_for_inode() function. Every inode storing function 
(there is no too much I think) should call 
jffs2_reserve_space_for_inode() with some extra information (inode 
number...).

jffs2_reserve_space() should use only ANYTHING_OTHER eraseblocks, as 
jffs2_reserve_space_for_inode() use only INODE_ONLY ones. If there is no 
free space in them it should use the usual technique to find a clean 
eraseblock and start to store the new node in it.

The generating of summary is also the task of 
jffs2_reserve_space_for_inode(), if the new inode (+summary) is not fit 
in the erase block, it will generates summary.

What do you think?

Regards,
Ferenc