linux-ext4 - Re: [PATCH] ext4: fake direct I/O mode for data=journal

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Message-ID: <20110815180311.GC5948@thunk.org>
Date:	Mon, 15 Aug 2011 14:03:11 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Jan Kara <jack@...e.cz>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: fake direct I/O mode for data=journal

On Mon, Aug 15, 2011 at 01:59:39PM +0200, Jan Kara wrote:
>   Why have you chosen to set O_DSYNC?

O_DSYNC was the closest thing to O_DIRECT, in that writes get forced
to disk after the write finished.  I did this fix a test failure
caused by missing direct I/O writes with a buffered read.  OTOH, there
is a performance cost in doing this since it will force a journal
commit after every write.  That's probably not acceptable.  Hmph.

>   Also what about reads? Mixing of buffered writes and direct IO reads will
> not work because filemap_write_and_write() does not write data block to the
> final location on disk but only to a journal.

Given that, we're probably better off forcing reads drop O_DIRECT as
well and drop O_DSYNC.

I'll change the patch and then give it a test.

     	    	      	       	       	 - Ted
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