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

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Date:	Thu, 12 Feb 2009 10:43:12 -0600
From:	Eric Sandeen <sandeen@...hat.com>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	Jan Kara <jack@...e.cz>, 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>
Subject: Re: ext2 + -osync: not as easy as it seems

Jens Axboe wrote:
> 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.

(sorry for chiming in so late)

Jens, isn't this just the way it's implemented today?  At some point
couldn't a barrier bio simply be a reordering barrier that the storage
can use when destaging the write cache, rather than the heavy-handed
flush-write-flush we have today?

I guess it's a question of the intended semantics of a barrier bio, vs.
today's implementation based on current hardware functionality...

-Eric
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