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

    > Alasdair G Kergon <> 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 ;-)

    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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