Re: JFFS2 losing files on mount

[David Woodhouse <dwmw2@infradead.org>]

On Thu, 2009-01-22 at 07:33 +0100, Ricard Wanderlof wrote:
> On Thu, 22 Jan 2009, David Woodhouse wrote:
> 
> >> We use JFFS2 on both an ARM9-platform and on our own CRIS platform. We
> >> never unmount the device in preparation for power down; when someone pulls
> >> the plug, the system just dies. While we do occasionally see warning
> >> messages about incomplete nodes during the next boot-up, we have never
> >> experienced anything as severe as you mention, and we have not lost any
> >> files on the JFFS2 filesystem either.
> >
> > Is this on NAND, or NOR flash? On NAND flash, you do actually have to
> > have non-buggy userspace; if you don't use sync() or fsync()
> > appropriately then there may be data which aren't yet flushed to the
> > flash.
> 
> To be more specific, on NOR flash we never say any messages about bad data 
> nodes. With NAND flashes we do. However, we have never experienced missing 
> files the way Marco seems to have.
> 
> What's the situation though, assuming one doesn't use sync()/fsync(),
> I can understand that data won't get flushed properly, but would that 
> result in a file being lost?

Not unless the file was only just created, and you lose power before
that creation hits the medium.

> In our case, the device may be powered down at any time. Let's assume an 
> application has a lot of data to write, using several write() calls. I can 
> envisage two scenarios:
> 1. Userspace app writes all data, but fails to call sync/fsync before
>     power fails.
> 2. Userspace app writes some of its data, but power fails before it
>     has time to write all.
> 
> In both cases I would imagine that the end result is the same - all data
> is not written to the flash because the plug is pulled. Considering the 
> data in the file, it doesn't matter if it wasn't written out by JFFS2 or 
> becuase the application died to early. Or is this a faulty assumption?

No, you're quite right.

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@intel.com                              Intel Corporation