linux-kernel - Re: ext2 + -osync: not as easy as it seems

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Message-ID: <20090114140802.GC30821@kernel.dk>
Date:	Wed, 14 Jan 2009 15:08:04 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Theodore Tso <tytso@....edu>,
	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Pavel Machek <pavel@...e.cz>,
	kernel list <linux-kernel@...r.kernel.org>, sandeen@...hat.com
Subject: Re: ext2 + -osync: not as easy as it seems

On Wed, Jan 14 2009, Jan Kara wrote:
> > I'm not sure what you mean; if the barrier operation isn't flushing
> > all of the caches all the way out to the iron oxide, it's not going to
> > be working properly no matter where it is being called, whether it's
> > in ext4_sync_file() or in jbd2's journal_submit_commit_record().
>   Well, I thought that a barrier, as an abstraction, only guarantees that
> any IO which happened before the barrier hits the iron before any IO which
> has been submitted after a barrier. This is actually enough for a
> journalling to work correctly but it's not enough for fsync() guarantees.
> But I might be wrong...

It also guarentees that when you get a completion for that barrier
write, it's on safe storage. Think of it as a flush-write-flush
operation, in the presence of write back caching.

-- 
Jens Axboe

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