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Re: dd PATCH: add conv=direct

From: Wim Coekaerts
Subject: Re: dd PATCH: add conv=direct
Date: Thu, 8 Apr 2004 18:42:44 -0700
User-agent: Mutt/

philip copeland did a whole set of patches for coreutils to allow
directio for both read write and mixed sizes even
the rpm is at
I think he took it up with the maintainers but so far had no luck

it makes sense to have this stuff, we use it a lot.


On Fri, Apr 09, 2004 at 10:37:37AM +1000, Anton Blanchard wrote:
> Hi,
> > OK, I can see that one.  But it seems like a pretty small benefit to me
> > -- CPU utilization is already really low.
> Maybe not to you but it does make a big difference on our 500 disk setup.
> At the moment we use dd to do an initial sniff, then ext3 utils to do
> O_DIRECT reads/writes. With O_DIRECT read/write in dd we could use it
> instead. (We are basically interested in IO performance that a database
> would see).
> > 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.
> When you start hitting memory bandwidth limits, O_DIRECT will help you.
> Sure it wont be an issue for your dd copy scenario, but I wanted to point
> out there are other valid uses for it.
> Anton
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