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Re: dd PATCH: add conv=direct

From: Anton Blanchard
Subject: Re: dd PATCH: add conv=direct
Date: Fri, 9 Apr 2004 10:37:37 +1000
User-agent: Mutt/


> 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.


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