linux-kernel - Re: EXT4-ish "fixes" in UBIFS

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Date:	Sun, 29 Mar 2009 15:54:40 +0300
From:	Artem Bityutskiy <dedekind@...dex.ru>
To:	Kyungmin Park <kmpark@...radead.org>
CC:	Artem.Bityutskiy@...ia.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: EXT4-ish "fixes" in UBIFS

Artem Bityutskiy wrote:
> Kyungmin Park wrote:
>> I also got these request. the file is empty at rename operatoin in
>> case of sudden power off.
>> they say it's different from jffs2. in case of jffs2, it points old
>> files even though power off.
> 
> Right, because JFFS2 is synchronous :-)
> 
>> then why is UBIFS different. fix it as before. I said it's not
>> filesystem bug. it's expected behaviors.
> 
> Right, this is what I've been always thinking. I've always been
> thinking the FS gives no guarantees, and if you want a 100%
> guarantee, use fsync() before renaming. Frankly, I still think
> so. But we'll make ext4-like changes in UBIFS as well to help
> the applications which do not do the sync.
> 
>> Frankly I'm not sure which one is better. how much filesystem support
>> it. but remember that application programmer also don't want to change
>> their application when filesystem is changed.
>> "The application is not changed, only filesystem is changed. so it's
>> filesystem problem, not us"
> 
> I hope Linux gurus will put it clearly after all - to fsync() or to
> not fsync(). We do need clear rules of the game. For now, I still
> assume the following:
> 
> 1. If applications want atomic update which gives 100% guarantee,
>   they should fsync before rename.
> 2. If the application does not use fsync, FS should try to minimize
>   the probability of data loss by running asynchronous write-back
>   on rename which unlinks a direntry.
> 3. All this performance vs. reliability hassle should be solved
>   by fixing the FS, by having good defaults, by having a
>   "fsync/not fsync" knobs in applications.
> 
> Indeed, people mostly talk about ext3, desktops, etc. But there
> is also the embedded world, where battery is removed randomly.

Let me elaborate why I tell about embedded. Looking into the
"Linux-2.6.29" thread, it _seems_ people assume that it is enough
if FS will start _asynchronous_ write-back after rename, so that
dirty data will not sit in the cache for long time. E.g., many
people are happy with ext3's 5 seconds. So for me it seems like
some people do not care about 100% atomicity guarantees, they are
fine with just low data loss probability.

So what I say, that in embedded we need 100% atomic updates,
because our power cuts may be frequent and random. And at this
moment only fsync() before rename may guarantee this.

And updating a file using truncate/rewrite does not guarantee
anything at all.

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)
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