Re: For UBIFS users: be aware of write-back!

Artem Bityutskiy
2008-10-03T07:45:46Z

On Fri, 2008-10-03 at 08:15 +0100, David Woodhouse wrote:
> On Wed, 2008-10-01 at 08:51 +0300, Artem Bityutskiy wrote:
> >
> > this e-mail informs about a quite important UBIFS feature which
> > have already confused many people.
> >
> > If you use UBIFS, and especially if you have been using JFFS2 before,
> > please, read this documentation entry:
> >
> > http://www.linux-mtd.infradead.org/doc/ubifs.html#L_writeback
> >
> > Please, remember that UBIFS is _asynchronous_ FS. JFFS2 was synchronous,
> > so expect some of your old applications which were fine on JFFS2 to be
> > not ready to handle power cuts correctly in case of UBIFS.
>
> JFFS2 wasn't synchronous on NAND. You still needed to use fsync() as
> appropriate to ensure that written data made it to the flash and not
> just to the write-buffer.

That's right, the above documentation even tells about this:

"In contrast, JFFS2 does not have write-back support and all the JFFS2
file system changes go the flash synchronously. Well, this is not
completely true and JFFS2 does have a small buffer of a NAND page size,
but it is small and we may treat JFFS2 as completely synchronous."

> Your list of things to think about is good, but it should be pointed out
> that it applies _generally_ to all systems and all file systems.

Right, agree. I'll amend the docs on the web site.