Re: jffs2

["Artem B. Bityuckiy" <abityuckiy@yandex.ru>]

Kornel Masłowski wrote:
> I'm not sure how jffs2 works.
> I have jffs2 on  8bit NAND flash (page 512 B).
> I want to append record 32B to the file on flash which current size is 
> 10kB.
> Flash device is almost empty.
> How many pages will be written by jffs2?
> 
> 
It is hard to calculate because of compression. Moreover, if your file 
was created by
seek to "nowhere" it will cosist of only one "hole" node and fit into 
one NAND page only.

If there is no compression, the exact number of pages also depends on 
how your file was created. Also, there is some platform dependency - 
does you platform has 4K RAM pages? - but this is minor since most 
platforms have 4K RAM pages.

If your 10K file was created by one go and there is no compression, the 
file will consist of 3 JFFS2 nodes (4K + 4K + 2K) and fit into 9 + 9 + 5 
= 23 NAND pages (4K data = 4 NAND pages and one is needed for node 
header; the rest of page won't be used since the next node isn't fit to it).

When you append 32 bytes, one more page will be used (24 total). But if 
your append just after 10K file creation (i.e, the JFFS2 write buffer 
hasn't yet been written to the NAND flash media), the 32B appendix node 
will be added to the 23-th page and there will be only 23 used pages.

If the compression is on, there ought to be fewer used NAND pages.

Ok, this is theory (I didn't check) and my own understanding of how 
JFFS2 works. I might make mistakes :-)

You may check this by creating/appending your file and then calculating 
the numbur of used pages. The nanddump utility may be used.

-- 
Best Regards,
Artem B. Bityuckiy,
St.-Petersburg, Russia.