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Date: Wed, 27 Jul 2011 17:05:14 -0400
From: Christoph Hellwig <hch@infradead.org>
To: Anand Avati <avati@gluster.com>
Cc: miklos@szeredi.hu, fuse-devel@lists.sourceforge.net,
        linux-fsdevel@vger.kernel.org, linux-nfs@vger.kernel.org
Subject: Re: [PATCH v2 2/2] fuse: permit O_DIRECT flag in open()
Message-ID: <20110727210514.GD9066@infradead.org>
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On Wed, Jul 27, 2011 at 03:56:19AM -0700, Anand Avati wrote:
> FUSE currently disallows O_DIRECT flag in open(). It is tricky dealing with
> setting/unsetting of O_DIRECT flag on an open file. There are applications
> (primarily VMs and databases) which open files with O_DIRECT flag. These
> applications do not work on FUSE due to this limitation.
> 
> The approach with this patch is to permit opens with O_DIRECT, but instead
> disable setting/unsetting of the O_DIRECT flag no matter how the file was
> opened. This limitation is for more practical than disallowing O_DIRECT
> altogether.

But it's also entirely incorrect.  Fix your userspace to have a proper
fallback if O_DIRECT opens fail.  It's a feature only supported by a few
filesystems, and very few operating systems.