On Sun, 21 Dec 2008 19:16:32 +0000, "Peter Grandi" <pg_mh@xxxxxxxxxx>
> > The drive itself may still re-order writes, thus can cause
> > corruption if halfway the power goes down. [ ... ] Barriers need
> > to travel all the way down to the point where-after everything
> > remains in-order. [ ... ] Whether the data has made it to the
> > drive platters is not really important from a barrier point of
> > view, however, iff part of the data made it to the platters, then
> > we want to be sure it was in-order. [ ... ]
> But this discussion is backwards, as usual: the *purpose* of any
> kind of barriers cannot be just to guarantee consistency, but also
> stability, because ordered commits are not that useful without
> commit to stable storage.
I do not see in what sense you mean "stability"? Stable as in BIBO or
Barriers are time-related. Once data is on storage, there is no relation
So I do not see how barriers help to "stabilize" storage.
Ordered commits is a strong-enough condition to ensure consistency in
the sense that
atomic transactions either made it to the disk completely or not at all.
> If barriers guarantee transaction stability, then consistency is
> also a consequence of serial dependencies among transactions (and
> as to that per-device barriers are a coarse and very underoptimal
Of course, the higher level should ensure that between transactions, the
(meta)data is always consistent.
In filesystem design, we see that some FS's decide to split metadata and
data in this regard.
> Anyhow, barriers for ordering only have been astutely patented
> quite recently:
> Amazing new from the patent office.y
Grand. Another case of no prior art. :-)