Re: [Xen-devel] [PATCH for-4.6] libxl, hotplug/Linux: default to phy backend for raw format file

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Jan 08, 2015 at 02:07:42PM +0000, Ian Campbell wrote:
> On Wed, 2014-11-26 at 16:55 +0000, Wei Liu wrote:
> > Modify libxl and hotplug script to allow raw format file to use phy
> > backend.
> > 
> > The block script now tests the path and determine the actual type of
> > file (block device or regular file) then use the actual type to
> > determine which branch to run.
> > 
> > With these changes, plus the current ordering of backend preference (phy
> > > qdisk > tap), we will use phy backend for raw format file by default.
> 
> http://lists.xen.org/archives/html/xen-devel/2012-04/msg00077.html
> includes (in the quotes, Stefano's reply is about something else but has
> conveniently trimmed most of the other uninteresting stuff):
>         use /dev/loop+blkback. This requires loop driver AIO and
>         O_DIRECT patches which are not (AFAIK) yet upstream.
> 
> and I have it in my mind that using /dev/loop+blkback is somehow unsafe,
> for reasons relating to crash consistency and the proper implementation
> (in /dev/loop, blkback is good I think) of barriers and such, e.g.
> relating to whether data is really on the platter or not when we've to
> the frontend that it is (which is critical for proper operation of
> journalling file systems).
> 
> It's entirely possible that I'm either plain wrong or a decade out of
> date on this though. CC-ing Konrad in case he has any insights as
> blkback maintainer (I think, MAINTAINERS doesn't have a specific entry)
> 

Too bad, as far as I can tell AIO and O_DIRECT are still missing in loop
device. I guess we will have to wait until those two things are
upstreamed.

Wei.

> > 
> > Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> > Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > Cc: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> > ---
> >  tools/hotplug/Linux/block  |   16 +++++++++-------
> >  tools/libxl/libxl.c        |    6 +++---
> >  tools/libxl/libxl_device.c |    2 ++
> >  tools/libxl/libxl_linux.c  |    6 +++---
> >  4 files changed, 17 insertions(+), 13 deletions(-)
> > 
> > diff --git a/tools/hotplug/Linux/block b/tools/hotplug/Linux/block
> > index da26e22..8d2ee9d 100644
> > --- a/tools/hotplug/Linux/block
> > +++ b/tools/hotplug/Linux/block
> > @@ -206,6 +206,13 @@ and so cannot be mounted ${m2}${when}."
> >  
> > 
> >  t=$(xenstore_read_default "$XENBUS_PATH/type" 'MISSING')
> > +p=$(xenstore_read "$XENBUS_PATH/params")
> > +mode=$(xenstore_read "$XENBUS_PATH/mode")
> > +if [ -b "$p" ]; then
> > +    truetype="phy"
> > +elif [ -f "$p" ]; then
> > +    truetype="file"
> > +fi
> 
> IIRC xend used to write file or phy to the type node as appropriate. I'm
> unsure if that was in some way better than this approach, and perhaps
> libxl doesn't even have the right information to hand.
> 
> Anyway, I just mention it for completeness, ignore it if you like.
> 
> Ian.

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