From mboxrd@z Thu Jan  1 00:00:00 1970
From: Phil Turmel <philip@turmel.org>
Subject: Re: RAID 5,6 sequential writing seems slower in newer kernels
Date: Wed, 2 Dec 2015 21:38:33 -0500
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To: Dallas Clement <dallas.a.clement@gmail.com>
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On 12/02/2015 09:33 PM, Dallas Clement wrote:

> I'm not sure that the sync=1 has any effect in this case where I've
> got direct=1 set (for non buffered I/O).  I think the sync=1 flag only
> matters for buffered I/O.  I really shouldn't be setting that flag at
> all.

It's substantially different from direct=1.  O_DIRECT just bypasses the
kernel's caches.  O_SYNC flushes the file data and filesystem metadata,
and kills the device caches and queues.