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

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Date:	Thu, 2 Apr 2009 18:24:28 -0700 (PDT)
From:	david@...g.hm
To:	Matthew Garrett <mjg59@...f.ucam.org>
cc:	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 Fri, 3 Apr 2009, Matthew Garrett wrote:

> On Thu, Apr 02, 2009 at 06:16:20PM -0700, david@...g.hm wrote:
>> On Fri, 3 Apr 2009, Matthew Garrett wrote:
>>
>>> On Thu, Apr 02, 2009 at 05:55:11PM -0700, david@...g.hm wrote:
>>>> On Fri, 3 Apr 2009, Matthew Garrett wrote:
>>>>> Then they shouldn't use a mail client that fsync()s.
>>>>
>>>> so they need to use one mail client when they want to have good battery
>>>> life and a different one when they are plugged in to power?
>>>
>>> They need to make a decision about whether they care about their mailbox
>>> being precisely in sync with their server or not, and either use a
>>> client that adapts appropriately or choose a client that behaves
>>> appropriately. It's certainly not the kernel's business.
>>
>> the kernel is not deciding this, the kernel would be implementing the
>> user's choice
>
> No it wouldn't. The kernel would be implementing an adminstrator's
> choice about whether fsync() is important or not. That's something that
> would affect the mail client, but it's hardly a decision based on the
> mail client. Sucks to be that user if they do anything involving mysql.

in the case of laptops, in 99+% of the cases the user and the 
administrator are the same person. in the other cases that's something the 
user should take up with the administrator, because the administrator can 
do a lot of things to the system that will affect the safety of their data 
(including loading a kernel that turns fsync into a noop, but more likely 
involving enabling or disabling write caches on disks)

>>> If you can demonstrate a real world use case where the hard drive
>>> (typically well under a watt of power consumption on modern systems)
>>> spindown policy will be affected sufficiently pathologically by a mail
>>> client that you lose an hour of battery life, then I'd rethink this. But
>>> mostly I'd conclude that this was an example of an inappropriate
>>> spindown policy.
>>
>> remember that the mail client was an example.
>>
>> you want another example, think of anything that uses sqlite (like the
>> firefox history stuff, although that was weakened drasticly due to the
>> ext3 problems).
>
> Benchmarks please.

if spinning down a drive saves so little power that it wouldn't make a 
significant difference to battery lift to leave it on, why does anyone 
bother to spin the drive down?

David Lang
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