Re: UBIFS robustness questions

[Artem Bityutskiy <dedekind1@gmail.com>]

On 07/24/2009 07:00 AM, Charles Manning wrote:
> This is probably documented somewhere but I could not find it...
>
> What operations in UBIFS are robust to power failure and which are not?

Hi, did you look through these:

http://www.linux-mtd.infradead.org/doc/ubifs.html#L_writeback
http://www.linux-mtd.infradead.org/doc/ubifs.html#L_writebuffer
http://www.linux-mtd.infradead.org/doc/ubifs.html#L_sync_exceptions
http://www.linux-mtd.infradead.org/faq/ubifs.html#L_empty_file

>
> I know for example that writing a file into flash does not mean it has been
> completely written to flash until after a sync, but what about other
> operations such as mv?
>
> The reasonn I'm asking this is that I want to be able to "hot-swap" a
> directory of files without losing any file state.

Err, if you do sync() and the like properly, you should not loose anything.

> What I'm considerings doing is something like:
>
> Start with ~/runtime having a sane set of files
>
> untar etc into ~/updated
> sync
> mv ~/updated ~/run-time
> sync
>
> What is unacceptable is that, at any time, a power failure/reboot results in
> ~/runtime having a non-sane set of files.

Err, this will just move "updated" to the "runtime" directory. Is this what
you mean? But the above must be safe.

> * Does the above sequence look safe?
> * Is the second sync required?

It is required if you want to make sure that the directory has really been renamed,
otherwise the renaming data will sit in the write-buffer for some time, and in case
of a power you end up with "updated" at the old place, but nothing should be
corrupted. IOW, you do not have to, but may want to.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)