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Re: dd PATCH: add conv=direct
From: |
Andy Isaacson |
Subject: |
Re: dd PATCH: add conv=direct |
Date: |
Wed, 7 Apr 2004 14:47:27 -0500 |
User-agent: |
Mutt/1.4.1i |
On Wed, Apr 07, 2004 at 12:34:55PM -0700, Andrew Morton wrote:
> Andy Isaacson <address@hidden> 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
- dd PATCH: add conv=direct, Andy Isaacson, 2004/04/07
- Re: dd PATCH: add conv=direct, Andrew Morton, 2004/04/07
- Re: dd PATCH: add conv=direct, Bruce Allen, 2004/04/07
- Re: dd PATCH: add conv=direct, Andy Isaacson, 2004/04/07
- Re: dd PATCH: add conv=direct, Andrew Morton, 2004/04/07
- Re: dd PATCH: add conv=direct, Andy Isaacson, 2004/04/07
- Re: dd PATCH: add conv=direct, Andrew Morton, 2004/04/07
- Re: dd PATCH: add conv=direct,
Andy Isaacson <=
- Re: dd PATCH: add conv=direct, Andrew Morton, 2004/04/07
- Re: dd PATCH: add conv=direct, Andy Isaacson, 2004/04/07
- Re: dd PATCH: add conv=direct, Valdis . Kletnieks, 2004/04/08
- Re: dd PATCH: add conv=direct, Bruce Allen, 2004/04/08
- Re: dd PATCH: add conv=direct, Paul Eggert, 2004/04/08
- Re: dd PATCH: add conv=direct, Jim Meyering, 2004/04/08
- Re: dd PATCH: add conv=direct, Paul Eggert, 2004/04/08
- Re: dd PATCH: add conv=direct, Paul Jarc, 2004/04/08
- Re: dd PATCH: add conv=direct, Jim Meyering, 2004/04/08
- Re: dd PATCH: add conv=direct, Philippe Troin, 2004/04/08