Hello,

On Sun, 21 Dec 2008 19:16:32 +0000, "Peter Grandi" [off-list ref]
said:
 

quoted

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
non-volatile?

Barriers are time-related. Once data is on storage, there is no relation
with time.

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
design).

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:

  http://www.freshpatents.com/Transforming-flush-queue-command-to-memory-barrier-command-in-disk-drive-dt20070719ptan20070168626.php

Amazing new from the patent office.y

Grand. Another case of no prior art. :-)

Leon.