Re: [Qemu-devel] Re: [PATCH 2/2] Add flush=off parameter to -drive

[Jamie Lokier <jamie@shareable.org>]

Anthony Liguori wrote:
> On 05/11/2010 08:12 AM, Paul Brook wrote:
> >>>cache=always (or a more scary name like cache=lie to defend against
> >>>idiots)
> >>>
> >>>    Reads and writes are cached. Guest flushes are ignored.  Useful for
> >>>    dumb guests in non-critical environments.
> >>>       
> >>I really don't believe that we should support a cache=lie.  There are
> >>many other obtain the same results.  For instance, mount your guest
> >>filesystem with barrier=0.
> >>     
> >Ideally yes. However in practice I suspect this is still a useful option. 
> >Is
> >it even possible to disable barriers in all cases (e.g. NTFS under 
> >windows)?
> >
> >In a production environment it's probably not so useful - you're generally
> >dealing with long lived, custom configured guests.
> >
> >In a development environment the rules can be a bit different. For example 
> >if
> >you're testing an OS installer then you really don't want to be passing 
> >magic
> >mount options. If the host machine dies then you don't care about the 
> >state of
> >the guest because you're going to start from scratch anyway.
> >   
> 
> Then create a mount point on your host and mount the host file system 
> under that mount with barrier=0.

Two reasons that advice doesn't work:

1. It doesn't work in many environments.  You can't mount a filesystem
with barrier=0 in one place and barrier=1 on a different point, and
there's ofen only one host partition.

2. barrier=0 does _not_ provide the cache=off behaviour.  It only
disables barriers; it does not prevent writing to the disk hardware.

If you are doing a transient OS install, ideally you want an amount
equal to your free RAM not written to disk until the end.  barrier=0
does not achieve that.

> The problem with options added for developers is that those options are 
> very often accidentally used for production.

We already have risky cache= options.  Also, do we call fdatasync
(with barrier) on _every_ write for guests which disable the
emulated disk cache?

-- Jamie