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Date: Thu, 16 Jun 2011 12:34:04 +0300
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Subject: Re: [ANNOUNCE] Native Linux KVM tool v2
From: Pekka Enberg <penberg@kernel.org>
To: Christoph Hellwig <hch@infradead.org>
Cc: Anthony Liguori <anthony@codemonkey.ws>, Alexander Graf <agraf@suse.de>,
        Prasad Joshi <prasadjoshi124@gmail.com>, Avi Kivity <avi@redhat.com>,
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Hi Christoph,

On Thu, Jun 16, 2011 at 09:21:03AM +0300, Pekka Enberg wrote:
>> And btw, we use sync_file_range()

On Thu, Jun 16, 2011 at 12:24 PM, Christoph Hellwig <hch@infradead.org> wrote:
> Which doesn't help you at all.  sync_file_range is just a hint for VM
> writeback, but never commits filesystem metadata nor the physical
> disk's write cache.  In short it's a completely dangerous interface, and
> that is pretty well documented in the man page.

Doh - I didn't read it carefully enough and got hung up with:

    Therefore, unless the application is strictly performing overwrites of
    already-instantiated disk blocks, there are no guarantees that the data will
    be available after a crash.

without noticing that it obviously doesn't work with filesystems like
btrfs that do copy-on-write.

What's the right thing to do here? Is fdatasync() sufficient?

                        Pekka