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Date: Thu, 31 Mar 2016 00:55:29 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Liu Bo <bo.li.liu@oracle.com>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>,
        Christoph Hellwig <hch@infradead.org>, xfs@oss.sgi.com,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
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Subject: Re: fallocate mode flag for "unshare blocks"?
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On Wed, Mar 30, 2016 at 05:32:42PM -0700, Liu Bo wrote:
> Well, btrfs fallocate doesn't allocate space if it's a shared one
> because it thinks the space is already allocated.  So a later overwrite
> over this shared extent may hit enospc errors.

And this makes it an incorrect implementation of posix_fallocate,
which glibcs implements using fallocate if available.