linux-kernel - replace() system call needed (was Re: EXT4-ish "fixes" in UBIFS)

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Message-ID: <20090329122600.GA13737@elf.ucw.cz>
Date:	Sun, 29 Mar 2009 14:26:00 +0200
From:	Pavel Machek <pavel@....cz>
To:	Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: replace() system call needed (was Re: EXT4-ish "fixes" in UBIFS)

On Fri 2009-03-27 14:48:10, Artem Bityutskiy wrote:
> UBIFS has exactly the same properties like ext4 - in case
> of power cuts:
>
> 1. truncate/write/close leads to empty files
> 2. create/write/rename leads to empty files
>
> UBIFS is used in hand-held and and power-cuts are very
> often there, because users just remove battery often.
>
> I realize the "reality is different" argument, and already
> concluded that we need a similar changes as Theo has done
> for ext4:
> http://git.kernel.org/?p=linux/kernel/git/tytso/ext4.git;a=commitdiff;h=bf1b69c0db7f9b9d8f02e94d40b19fca8336b991
> http://git.kernel.org/?p=linux/kernel/git/tytso/ext4.git;a=commitdiff;h=f32b730a69bd56c5c9d704d8b75f03e90e290971
> http://git.kernel.org/?p=linux/kernel/git/tytso/ext4.git;a=commitdiff;h=8411e347c3306ed36b8ca88611bf5fbf4d27d705
>
> We have a problem that user-space people do not want to
> use 'fsync()', even when they are pointed to their code
> which is doing create/write/rename/close without fsync().

Well... they really don't want to spin the disk up for the
fsync(). I'm not sure if fsync() is really sensible operation to use
there.

> 1. truncate/write/close leads to empty files

this is buggy.

> 2. create/write/rename leads to empty files

...but this should not be. If we want to make that explicit, we should
provide "replace()" operation; where replace is rename that makes sure
that source file is completely on media before commiting the rename.

It is somehow similar to fsync()/rename(), but does not force disk
spin up immediately -- it only inserts "barrier" between data blocks
and rename. (And yes, it should be implemented as fsync()+rename() for
filesystems like xfs. It can be implemented as plain rename for ext3
and ext4 after the fixes...)

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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