On Mon, Jan 25, 2010 at 22:33:20 nixlists wrote:
> > On Mon, Jan 25, 2010 at 04:35:48PM -0500, nixlists wrote:
> >> On Mon, Jan 25, 2010 at 4:12 PM, Marco Peereboom
> >> <[email protected]>
> wrote:
> >> > You are positively ignorant. No need to regurgitate this all
> >> > over
again. Take your toy mail implementation and enjoy your hair.
> >>
> >> You are still refusing to give a direct answer to a direct
> >> question.
How's that not ignorant? I wonder why that might be... All this
> >> "well, we can't really tell what the hardware may do" crap isn't
> >> enough. Perhaps you don't have an answer....
> >
> > Y'know, if you don't get the fact that the answer you're being given
> > is that, ultimately, there really *isn't* an answer, you need some
> > more zen in your diet.
>
> No, I've been given an answer for the RAID controllers (and even that
> was nebulous), now let's hear it for the SATA.
>
> Again. no write-back cache anywhere, no softupdates, no async mounts,
> does the guarantee in the rename(2) apply to this case?
>
> If it does, then say so . If it doesn't, then say so (and change the
> man page, maybe?).
Let's try this again. If power is removed during the physical writing
of a given byte to the actual platter:
1) That byte will not be correctly written.
2) The fact that it was not correctly written cannot be logged.
3) The fact that previous bytes WERE correctly written MIGHT
have been logged, but exactly what happened to the one that got
jacked up will not be logged.
This is not about software. Or about cache settings. This is about
electricity and magnetic domains on platters. There is no such thing
as a driver that can really get behind this anymore than there is a
driver that can change the output voltages on an ethernet card.
So guarantees of integrity made by drivers and functions are always
conditional; they are predicated on success at the electromagnetic
level. Fault tolerant DBMSs cheat by writing a lot to their log files
both before and after the fact of writing the "data". There is a
substantial time penalty for this.
The DBMS kind of guarantee does not come from anything like a normal
device driver. It comes from a DBMS, or else an O/S that is
written like a DBMS, and which will incur the aforementioned penalty.
If this is not clear, then more thinking about the heads and the
little currents flowing through them over the oxide layer is needed.
--
Edward Ahlsen-Girard
Ft Walton Beach, FL
