linux-kernel - Re: Ext4 and the "30 second window of death"

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Message-ID: <20090403010901.GB10545@srcf.ucam.org>
Date:	Fri, 3 Apr 2009 02:09:01 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	david@...g.hm
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Theodore Tso <tytso@....edu>,
	Sitsofe Wheeler <sitsofe@...oo.com>,
	"Andreas T.Auer" <andreas.t.auer_lkml_73537@...us.ath.cx>,
	Alberto Gonzalez <info@...bu.es>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Ext4 and the "30 second window of death"

On Thu, Apr 02, 2009 at 05:59:53PM -0700, david@...g.hm wrote:

> is laptop mode
> 
> A. "write everything now, don't delay writes" in the hope that the drive 
> will be idle enough later to spin down

laptop-mode doesn't delay writes. Ever.

> or
> 
> B. "delay all writes until later, then when the drive wakes up do all 
> pending writes at that time" so that the drive can go to sleep in the 
> meantime?

Yes.

> I've heard things in these threads that would indicate both behaviors.

The code's pretty trivial. The only real functional differences 
laptop-mode brings are to write out all dirty pages (rather than just 
writing down to the watermark) and to call sys_sync() a few seconds 
after the last thing that hit disk rather than being satisfied from 
cache. It's entirely a mechanism to opportunistically take advantage of 
the disk being spun up.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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