Messages in this thread | | | Date | Wed, 7 Apr 2004 14:47:27 -0500 | From | Andy Isaacson <> | Subject | Re: dd PATCH: add conv=direct |
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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.
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