Re: A question about UBIFS

[Adrian Hunter <ext-adrian.hunter@nokia.com>]

Zhao Forrest wrote:
> Hi Artem,

Artem is on holiday til next week.

> I have a basic question about NAND flash page update in UBIFS/UBI,
> take the following as an example:
> 1 a NAND flash with page size of 4KB, erase block size of 256KB, so
> there're 64 pages within an erase block
> 2 page 0 - page 10 has invalid data content, page 11 - page 63 has
> valid data content from FS's point of view
> 3 UBIFS attempts to update(or rewrite) page 11

UBIFS does out-of-place updates. It never tries to update or rewrite
page 11. (Well, except in very rare circumstances that are not worth
going into)

> My question is how page 11 is updated by UBIFS/UBI?

As far as UBIFS goes, you need to learn about wandering trees and
garbage collection.

> I only read the design docs of UBI and guess that for this particular
> case a new physical eraseblock is got by UBI, then new content of page
> 11 is written to page 11 of new physical eraseblock, page 12 - page 63
> in original physical eraseblock are copied to new physical eraseblock,
> lastly the mapping between logical eraseblock and physical eraseblock
> is updated. Am I right?

Not for UBIFS.

> I know I omit many details in UBIFS/UBI for this updating operation.
> I'd like to know the cost of rewrite in UBIFS/UBI.

UBIFS rewrite is typically better than even JFFS2 - see
http://osl.sed.hu/wiki/ubifs/index.php/IOzone

Of course that assumes you are sync.ing the data.  UBIFS has writeback
which means rewrites will be much faster if you do not need to have
the data sync'ed.