Hello, On Sun, 21 Dec 2008 19:16:32 +0000, "Peter Grandi" <pg_mh@xxxxxxxxxx> said: > > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
