Subject: UBIFS and small dynamic files

[Jon Povey]

You need to rewrite files with safe POSIX semantics.
It is not crash-safe to rewrite the target file directly (but many
apps are naive and do this in an unsafe way)

The right way to do it is write a new file on the same filesystem,
fsync() it, then rename it over the target. The rename is atomic
so after a crash you either get the old file or the new one.

I had a quick google for the authoritative sequence but I couldn't
find and can't remember it exactly. I think it might involve an fsync
on the directory handle too if you want to be sure the new file has
arrived on flash. This avoids syncing the entire filesystem
or mounting it -o sync.

linux-mtd-bounces at lists.infradead.org wrote:
> Ahoi,
>
> we are having trouble with the side effects of the UBIFS write buffer
> described in
> http://www.linux-mtd.infradead.org/doc/ubifs.html#L_writebuffer
> We have certain config files that are quite small and get
> rewritten very
> frequently by their applications.
> Matchbox-panels' ~/.matchbox/mbdock.session is a fine example
> as it gets
> emptied on a power cut or hard reset resulting in an empty
> and therefore
> useless panel on the next start.
>
> The document above mentions the '-o sync' option which disables the
> write buffer on a partition, but this is not an option for us because
> we do not want to reduce the write performance. And there is
> also the issue
> of a synchronously mounted UBIFS that can make things even
> worse in case
> of power cuts.
>
>
>
> Has there been any further development to these issues since 2008?
>
>

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--
Jon Povey
jon.povey at racelogic.co.uk