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Date:   Thu, 3 Dec 2020 15:50:43 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     David Howells <dhowells@redhat.com>
Cc:     Christoph Hellwig <hch@infradead.org>, jlayton@redhat.com,
        dchinner@redhat.com, linux-fsdevel@vger.kernel.org,
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Subject: Re: Problems doing DIO to netfs cache on XFS from Ceph
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On Thu, Dec 03, 2020 at 02:10:56PM +0000, David Howells wrote:
> Note that I'm only doing async DIO reads and writes, so I was a bit surprised
> that XFS is doing a writeback at all - but I guess that IOCB_DIRECT is
> actually just a hint and the filesystem can turn it into buffered I/O if it
> wants.

That's almost the exact opposite of what is going on.  XFS sees that
you're going to do an O_DIRECT read, so it writes back the dirty memory
that's currently in the page cache so that your read doesn't read stale
data from disk.