Re: dd PATCH: add conv=direct

[Andy Isaacson <adi@hexapodia.org>]

On Wed, Apr 07, 2004 at 12:34:55PM -0700, Andrew Morton wrote:
> Andy Isaacson <adi@hexapodia.org> wrote:
> > Would there be any reason to allow O_DIRECT on the read side?
> 
> Sure.  It saves CPU,

OK, I can see that one.  But it seems like a pretty small benefit to me
-- CPU utilization is already really low.

> avoids blowing pagecache,

Um, that sounds like a bad idea to me.  It seems to me it's the kernel's
responsibility to figure out "hey, looks like a streaming read - let's
not blow out the buffer cache trying to hold 20GB on a 512M system."  If
you're saying that the kernel guys have given up and the established
wisdom is now "you gotta use O_DIRECT if you don't want to throw
everything else out due to streaming data", well... I'm disappointed.

> just as with O_DIRECT writes.

Wouldn't opening both if= and of= with O_DIRECT turn dd into a
synchronous copier?  That would really suck in the
"dd if=/dev/hda1 of=/dev/hdc1" case.  With buffer cache doing
readahead, that command can get, say, 40MB/s read and 40MB/s write;
with synch read and synch write, it would drop to 40MB/s read+write,
assuming that block sizes are big enough to amortize seek overhead.

Having O_DIRECT on just of=, I think you can get back to 40MB/s+40MB/s.

I claim that O_DIRECT on of= is important because you just plain *can
not* do the minimal-sized IDE block scrub without it.  I don't yet see a
similar benefit to O_DIRECT on if= side.

-andy