[dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

[Alasdair G Kergon]

On Tue, May 29, 2007 at 11:25:42AM +0200, Stefan Bader wrote:
> doing a sort of suspend, issuing the
> barrier request, calling flush to all mapped devices and then wait for
> in-flight I/O to go to zero? 

Something like that is needed for some dm targets to support barriers.
(We needn't always wait for *all* in-flight I/O.)
When faced with -EOPNOTSUP, do all callers fall back to a sync in
the places a barrier would have been used, or are there any more
sophisticated strategies attempting to optimise code without barriers?

> I am not a hundred percent sure about
> that but I think that just passing the barrier flag on to mapped
> devices might in some (maybe they are rare) cases cause a layer above
> to think all data is on-disk while this isn't necessarily true (see my
> previous post). What do you think?

An efficient I/O barrier implementation would not normally involve
flushing AFAIK: dm surely wouldn't "cause" a higher layer to assume
stronger semantics than are provided.
 
Alasdair
-- 
agk at redhat.com