DPT controller?

[Thomas Hoberg]

In article <1991May22.010734.15694 at nuchat.sccsi.com>, steve at nuchat.sccsi.com (Steve Nuchia) writes:
|> In article <31 at metran.UUCP> jay at metran.UUCP (Jay Ts) writes:
|> >I have been wondering under what conditions the DPT and other caching disk
|> >controllers are really effective enough to be worth the extra price and
|> ...
|> >cache on the controller is only about 10-15%, even if it is maxed out at
|> 
|> In an ideal world (ie, one in which you have source) you can make
|> certain trade-offs in software, where they belong, and hardware hacks
|> like caching controllers become completely irrelevant.  Without the
|> ability to make those trade-offs in software, sometimes hardware
|> is the only answer.
|> 
|> In sysV there are a number of places where synchronous writes to
|> the disk are forced.  Number one on my personal list of pet peeves
|> is unlink(2), for instance.  Using a non-write-through caching controller
|> will make rm -rf /usr/spool/news run as fast as it should instead of
|> taking about 50 ms per article.

It's about time USL implemented a NEWS file system (e.g. mount -f dispensible
/usr/news).

|> 
|> If you wanted to make the trade-off of non-synchrounous inode updates
|> for faster directory operations, and you don't have source, the only
|> way to do it is in hardware.  Depending on your jop mix it may be worth it.

Does anybody do a [E]ISA board with a battery buffered write cache?

|> -- 
|> Steve Nuchia	      South Coast Computing Services      (713) 964-2462
|> 	"Innocence is a splendid thing, only it has the misfortune
|> 	 not to keep very well and to be easily misled."
|> 	    --- Immanuel Kant,  Groundwork of the Metaphysic of Morals

Does the file system hardening (the forced synchronous writes) actually work
failsafe? I mean power can get lost at any time, even in the middle of a sector
write. A power supply can usually supply DC current for a certain period of time
after AC input is lost (right?). Does this always/usually suffice to finish the
sector write? I think the power fail line of a PC power supply triggers an NMI
on the motherboard. This should keep any further write requests from being
started, right? Basically, what I am asking: Are sector writes atomic, or have I
just been lucky? After all, if disk inode updates can be incomplete due to
partially written sectors, doesn't that invalidate file system hardening?

-- tom 
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