linux-kernel - Re: Disabling in-memory write cache for x86-64 in Linux II

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Message-ID: <20131025091842.GA28681@thunk.org>
Date:	Fri, 25 Oct 2013 05:18:42 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	"Artem S. Tashkinov" <t.artem@...os.com>
Cc:	torvalds@...ux-foundation.org, fengguang.wu@...el.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: Disabling in-memory write cache for x86-64 in Linux II

On Fri, Oct 25, 2013 at 08:30:53AM +0000, Artem S. Tashkinov wrote:
> My feeling is that vm.dirty_ratio/vm.dirty_background_ratio should _not_ be
> percentage based, 'cause for PCs/servers with a lot of memory (say 64GB or
> more) this value becomes unrealistic (13GB) and I've already had some
> unpleasant effects due to it.

What I think would make sense is to dynamically measure the speed of
writeback, so that we can set these limits as a function of the device
speed.  It's already the case that the writeback limits don't make
sense on a slow USB 2.0 storage stick; I suspect that for really huge
RAID arrays or very fast flash devices, it doesn't make much sense
either.

The problem is that if you have a system that has *both* a USB stick
_and_ a fast flash/RAID storage array both needing writeback, this
doesn't work well --- but what we have right now doesn't work all that
well anyway.

						- Ted
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