Date: Fri, 1 Apr 2016 17:27:45 +1100
From: Dave Chinner <david@fromorbit.com>
To: Jens Axboe <axboe@fb.com>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-block@vger.kernel.org
Subject: Re: [PATCHSET v3][RFC] Make background writeback not suck
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On Thu, Mar 31, 2016 at 09:25:33PM -0600, Jens Axboe wrote:
> On 03/31/2016 06:46 PM, Dave Chinner wrote:
> >>>virtio in guest, XFS direct IO -> no-op -> scsi in host.
> >>
> >>That has write back caching enabled on the guest, correct?
> >
> >No. It uses virtio,cache=none (that's the "XFS Direct IO" bit above).
> >Sorry for not being clear about that.
> 
> That's fine, it's one less worry if that's not the case. So if you
> cat the 'write_cache' file in the virtioblk sysfs block queue/
> directory, it says 'write through'? Just want to confirm that we got
> that propagated correctly.

No such file. But I did find:

$ cat /sys/block/vdc/cache_type 
write back

Which is what I'd expect it to safe given the man page description
of cache=none:

	Note that this is considered a writeback mode and the guest
	OS must handle the disk write cache correctly in order to
	avoid data corruption on host crashes.

To make it say "write through" I need to use cache=directsync, but
I have no need for such integrity guarantees on a volatile test
device...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com