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Subject: RE: Subject: UBIFS and small dynamic files
From: Artem Bityutskiy <dedekind1@gmail.com>
To: Jon Povey <Jon.Povey@racelogic.co.uk>
Date: Fri, 28 Oct 2011 17:03:16 +0300
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On Mon, 2011-10-24 at 04:21 +0100, Jon Povey wrote:
> 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)
>=20
> 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.
>=20
> 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.

You can set per-file "sync" attribute as well.

Someone can also implement something similar as ext4 has in UBIFS: we
can sync when the last reference to the file descriptor is dropped. I
also described this in the MTD web site.

--=20
Best Regards,
Artem Bityutskiy

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