linux-kernel - Re: Increase dirty_ratio and dirty_background

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Date:	Fri, 09 Jan 2009 22:34:31 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	david@...g.hm
Cc:	Jan Kara <jack@...e.cz>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Mason <chris.mason@...cle.com>,
	David Miller <davem@...emloft.net>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, npiggin@...e.de
Subject: Re: Increase dirty_ratio and dirty_background_ratio?

On Fri, 2009-01-09 at 14:31 -0800, david@...g.hm wrote:

> for that matter, it's not getting to where it makes sense to have wildly 
> different storage on a machine
> 
> 10's of GB of SSD for super-fast read-mostly
> 100's of GB of high-speed SCSI for fast writes
> TB's of SATA for high capacity
> 
> does it make sense to consider tracking the dirty pages per-destination so 
> that in addition to only having one process writing to the drive at a time 
> you can also allow for different amounts of data to be queued per device?
> 
> on a machine with 10's of GB of ram it becomes possible to hit the point 
> where at one point you could have the entire SSD worth of data queued up 
> to write, and at another point have the same total amount of data queued 
> for the SATA storage and it's a fraction of a percent of the size of the 
> storage.

That's exactly what we do today. Dirty pages are tracked per backing
device and the writeback cache size is proportionally divided based on
recent write speed ratios of the devices.
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