linux-kernel - Re: Linux 2.6.29

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Date:	Thu, 2 Apr 2009 10:04:29 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	David Rees <drees76@...il.com>, Janne Grunau <j@...nau.net>,
	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>,
	Theodore Tso <tytso@....edu>, Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

On Thu, 2 Apr 2009, Linus Torvalds wrote:
> 
> On Thu, 2 Apr 2009, Andrew Morton wrote:
> > 
> > A suitable design for the streaming might be, every 4MB:
> > 
> > - run sync_file_range(SYNC_FILE_RANGE_WRITE) to get the 4MB underway
> >   to the disk
> > 
> > - run fadvise(POSIX_FADV_DONTNEED) against the previous 4MB to
> >   discard it from pagecache.
> 
> Here's an example. I call it "overwrite.c" for obvious reasons.

Oh, except my example doesn't do the fadvise. Instead, I make sure to 
throttle the writes and the old range with

	SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE|SYNC_FILE_RANGE_WAIT_AFTER

which makes sure that the old pages are easily dropped by the VM - and 
they will be, since they end up always being on the cold list.

I _wanted_ to add a SYNC_FILE_RANGE_DROP but I never bothered because this 
particular load it didn't matter. The system was perfectly usable while 
overwriting even huge disks because there was never more than 8MB of dirty 
data in flight in the IO queues at any time.

		Linus
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