Subject: Re: True  fsync() in Linux (on IDE)
From: Chris Mason <mason@suse.com>
To: Peter Zaitsev <peter@mysql.com>
Cc: Hans Reiser <reiser@namesys.com>, Jens Axboe <axboe@suse.de>,
       Linux Kernel <linux-kernel@vger.kernel.org>
In-Reply-To: <1079724411.2576.178.camel@abyss.local>
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Date: Fri, 19 Mar 2004 15:23:53 -0500
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On Fri, 2004-03-19 at 14:26, Peter Zaitsev wrote:
> On Fri, 2004-03-19 at 05:52, Chris Mason wrote:
> 
> 
> > I am listening to Peter, Jens and I have spent a significant amount of
> > time on this code.  We can go back and spend many more hours testing and
> > debugging the 2.4 changes, or we can go forward with a very nice
> > solution in 2.6.
> > 
> > I'm planning on going forward with 2.6
> 
> Chris, Hans
> 
> It is great to hear this is going to be fixed in 2.6, however it is
> quite a pity we have a real mess with this in  2.4 series. 
> 
It is indeed.

> Resuming what I've heard so far it looks like it depends on:
> 
> - If it is fsync/O_SYNC or O_DIRECT   (which user would expect to have
> the same effect in this respect.
> - It depends on kernel version. Some vendors have some fixes, while
> others do not have them.
> - It depends on hardware - if it has write cache on or off 
> - It depends on type of write (if it changes mata data or not)
> - Finally it depends on file system and even journal mount options
> 
All of the above is correct.

> Just curious does at least Asynchronous IO have the same behavior as
> standard IO ? 
> 

For the suse patch, yes.  If it triggers a commit, you get a cache
flush.

> 
> All of these makes it extremely hard to explain what do users need in
> order to get durability for their changes, while preserving performance.
> 
> Furthermore as it was broken for years I expect we'll have people which
> developed things with fast fsync() in mind, who would start screaming
> once we have real fsync() 
> 
> (see my mail about Apple actually disabling cache flush on fsync() due
> to this reason) 

These are all difficult issues.  I wish I had easier answers for you,
hopefully we can get it all nailed down in 2.6 for starters.

-chris


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