linux-kernel - Re: [ANNOUNCE] Native Linux KVM tool v2

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Date:	Thu, 16 Jun 2011 12:34:04 +0300
From:	Pekka Enberg <penberg@...nel.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Anthony Liguori <anthony@...emonkey.ws>,
	Alexander Graf <agraf@...e.de>,
	Prasad Joshi <prasadjoshi124@...il.com>,
	Avi Kivity <avi@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Sasha Levin <levinsasha928@...il.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Asias He <asias.hejun@...il.com>,
	Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [ANNOUNCE] Native Linux KVM tool v2

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@...radead.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
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