linux-ext4 - Re: [PATCH] ext3,4:fdatasync should skip metadata writeout

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Message-Id: <20071115185919.7df4cda9.akpm@linux-foundation.org>
Date:	Thu, 15 Nov 2007 18:59:19 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] ext3,4:fdatasync should skip metadata writeout

On Fri, 16 Nov 2007 11:47:27 +0900 Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp> wrote:

> Currently fdatasync is identical to fsync in ext3,4.
> I think fdatasync should skip journal flush in data=ordered and data=writeback mode
> because this syscall is not required to synchronize the metadata.

I suppose so.  Although one wonders what earthly point there is in syncing
a file's data if we haven't yet written out the metadata which is required
for locating that data.

IOW, fdatasync() is only useful if the application knows that it is overwriting
already-instantiated blocks.

In which case it might as well have used fsync().  For ext2-style filesystems,
anyway.

hm.  It needs some thought.
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