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SubjectRe: "raw" block devices?
On Thu, 17 Oct 1996, Linus Torvalds wrote:

> On Thu, 17 Oct 1996, Ingo Molnar wrote:
> > >
> > > Sure, there are old-fashioned databases that think they can do a better job
> > > of it than the kernel does. They are usually wrong, I suspect. They are using
> > > raw devices more for historical reasons than anything else, and they could
> > > just as well use a filesystem.
> >
> > [yes, raw devices are a hack, still RDBMS ppl use it because:]
> >
> > one not-so obvious problem is that an RDBMS >has< to implement a
> > write-cache for itself. Thus if the block device would be buffered too (in
> > the kernel), then we had double buffering. [as it is buffered now]
> Not strictly true. Yes, there are ordering constraints, but you can handle
> them on a filesystem too (and people do). A raw device gives you more
> low-level control, but on the other hand it _also_ results in less chance for
> the OS to optimize data transfers for those cases where the optimizations
> would be valid.
> You can handle write ordering by using a log-based database (never overwrite
> any old data, so write ordering doesn't matter), and do a "fsync()" on the
> file when you commit. Voila, you just got guarantees about the file being
> on disk (assuming fsync() works as it is supposed to), and it doesn't
> matter if the OS decided it needed to write out part of the data earlier
> (in fact, that's only good for overlapping IO and calculations).

For example, RDB/VMS uses fairly ordinary files that you could just as
well access with normal QIO or RMS calls if you knew the internal
structure. I think it just maintains tight control over the ordering of
its writes when that's necessary. (Of course, VMS gives it tools to do
that, or this wouldn't work.) Digital did a *lot* of work to discover
where the order-dependencies are in their product, and it paid off.
(Trouble is, they got strapped for cash, so now it's paying off for Oracle

Mark H. Wood, Lead System Programmer MWOOD@INDYVAX.IUPUI.EDU
Those who will not learn from history are doomed to reimplement it.

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