On Monday May 28, [email protected] wrote:
> On Mon, May 28, 2007 at 11:30:32AM +1000, Neil Brown wrote:
> >
> > Thanks everyone for your input. There was some very valuable
> > observations in the various emails.
> > I will try to pull most of it together and bring out what seem to be
> > the important points.
> >
> >
> > 1/ A BIO_RW_BARRIER request should never fail with -EOPNOTSUP.
>
> Sounds good to me, but how do we test to see if the underlying
> device supports barriers? Do we just assume that they do and
> only change behaviour if -o nobarrier is specified in the mount
> options?
>
What exactly do you want to know, and why do you care?
The idea is that every "struct block_device" supports barriers. If the
underlying hardware doesn't support them directly, then they get
simulated by draining the queue and issuing a flush.
Theoretically there could be devices which have a write-back cache
that cannot be flushed, and you couldn't implement barriers on such a
device. So throw it out and buy another?
As far as I can tell, the only thing XFS does differently with devices
that don't support barriers is that it prints a warning message to the
kernel logs. If the underlying device printed the message when it
detected that barriers couldn't be supported, XFS wouldn't need to
care at all.
NeilBrown
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