Re: [Xen-devel] new knob to tweak caching mode for backends

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 27 May 2014, Jim Fehlig wrote:
> Olaf Hering wrote:
> > Currently libxl (and xend) has no knob to control cache mode of backend
> > driver for block devices. libvirt and qemu have:
> > cache=off|none|directsync|writeback|unsafe|writethrough.
> >
> > The xen qdisk driver in qemu defaults to "writeback". If the diskspec in
> > domUâcfg has 'direct-io-safe' then qdisk will default to directsync AIO.
> >
> > I think these defaults are fine as they provide some sort of data
> > integrity.
> >
> >
> > But there is one issue: all the flushing thats going on during guest
> > triggered writes does slows down the guest. There should be a knob to
> > skip the regular flushes on the host.  If the given backing file will
> > contain throw-away data its up the the admin to make this decision.
> >   
> 
> ACK to the idea of admin control of cache mode.  As you say, admins
> already enjoy this capability with libvirt and qemu.

There is nothing wrong with having advanced options for people that know
what they are doing and/or they are happy with taking risks and loosing
data. What's important is not to rely on these "advanced options" to get
a decent default configuration.

So I think that as long as we keep the default behaviour sane, we can
expose whatever advanced options we like.

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