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Date: Fri, 16 Mar 2001 12:11:47 +0000
From: "Stephen C. Tweedie" <sct@redhat.com>
To: Tom Vier <thomassr@erols.com>
Cc: Denis Perchine <dyp@perchine.com>, linux-kernel@vger.kernel.org,
        Stephen Tweedie <sct@redhat.com>
Subject: Re: O_DSYNC flag for open
Message-ID: <20010316121147.B1771@redhat.com>
In-Reply-To: <01031013035702.00608@dyp.perchine.com> <20010314222642.A19634@zero>
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In-Reply-To: <20010314222642.A19634@zero>; from thomassr@erols.com on Wed, Mar 14, 2001 at 10:26:42PM -0500
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Hi,

On Wed, Mar 14, 2001 at 10:26:42PM -0500, Tom Vier wrote:
> fdatasync() is the same as fsync(), in linux.

No, in 2.4 fdatasync does the right thing and skips the inode flush if
only the timestamps have changed.

> until fdatasync() is
> implimented (ie, syncs the data only)

fdatasync is required to sync more than just the data: it has to sync
the inode too if any fields other than the timestamps have changed.
So, for appending to files or writing new files from scratch, fsync ==
fdatasync (because each write also changes the inode size).  Only for
updating existing files in place does fdatasync behave differently.

> #ifndef O_DSYNC
> # define O_DSYNC O_SYNC
> #endif

2.4's O_SYNC actually does a fdatasync internally.  This is also the
default behaviour of HPUX, which requires you to set a sysctl variable
if you want O_SYNC to flush timestamp changes to disk.

Cheers,
 Stephen
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