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SubjectRe: [PATCH 6/9] device-mapper snapshot: barriers not supported
On 2006-01-22T21:41:11, Andrew Morton <akpm@osdl.org> wrote:

> Alasdair G Kergon <agk@redhat.com> wrote:
> >
> > The snapshot and origin targets are incapable of handling barriers and
> > need to indicate this.
> >
> > ...
> >
> > + if (unlikely(bio_barrier(bio)))
> > + return -EOPNOTSUPP;
> > +
>
> And what was happening if people _were_ sending such BIOs down? Did it all
> appear to work correctly? If so, will this change cause
> currently-apparently-working setups to stop working?

Filesystems basically disable using barriers on a device which doesn't
support them, which is indicated by -EOPNOTSUPP. Barriers are allowed to
fail in such fashion.

Now the interesting question is what happens when barriers are suddenly
verboten on a stack which used to support them - because the new mapping
doesn't support it _anymore_. Hrm. _Should_ work, but probably not
tested much ;-)


Sincerely,
Lars Marowsky-Brée

--
High Availability & Clustering
SUSE Labs, Research and Development
SUSE LINUX Products GmbH - A Novell Business -- Charles Darwin
"Ignorance more frequently begets confidence than does knowledge"

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