Re: UBI: question about ubi_leb_change

[Nancy <nancydreaming@gmail.com>]

> Hi,
>
> On Thu, 2008-03-06 at 20:26 +0800, Nancy wrote:
> > Hi,
> >     As I know, ubi_leb_change only accept a whole LEB data as one of
> > its parameter.
> Correct.
>
> >     If file system modify a little data which in a old mapped LEB, it
> > needs to read the whole old mapped LEB data in its buffer first, then
> > do the little modification, finally deliver to ubi_leb_change?  Is
> > that right?
> This is what can be done. But file systems should not normally do this,
> because it is inefficient. It is more efficient to do out-of-place
> updates, which means writing data to somewhere else (a free LEB), and
> treat the older data as obsolete. This is what JFFS2 and UBIFS do.
>
> UBIFS does use the in-place update method you have described, but very
> rarely. It uses it only when the FS is nearly full, and only as
> last-resort method of updating the FS index, when out-of-place updates
> are impossible.

Thank you for your information!  It do help a lot !

> >     when remove files, there's only inodes .....some file system meta
> > data changed,
> > but the real file data still exist which means the LEB still mapped.
> > Then when you write new data to this very LEB( but contains only
> > trash), maybe this time, your new data is a whole LEB size.  Still
> > goes the process:  read the mapped LEB old data to buffer,  replace
> > the old data with the new one in buffer, deliver to ubi_leb_change?
> > Really like that?
>
> If you have an LEB, and the contents of LEB is not needed, you should
> just unmap it and write your new data. No need to read it.
>

   Yes, but in FTL layer, it don't know whether the old mapped LEB is
useful or not until
the new mapped LEB which data stored in a write_cache have to be write
to PEB. The
write_cache maybe full filled with new data or just a little new data,
others have to be read from the old mapped PEB

http://lists.infradead.org/pipermail/linux-mtd/2008-January/020381.html
 suggest to use
ubi_leb_change.   Which force the write_cache to read a whole LEB(old
mapped) back first.
That waste so much time, in fact, can't bare, too slow!

    FTL is using in-place update,although write to different PEB,  the
unchange data have to be read back no matter those data is useful or
not ( cause in FTL layer it doesn't know it is useful or not)

    Or FTL can have a same idea like UBIFS, implement log to make
out-place change become reality .....  Is that possible?

    Thanks in advance!


--
Best wishes,
Nancy