linux-kernel - Re: [rfc] Ignore Fsync Calls in Laptop

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Message-ID: <alpine.DEB.2.02.1105301535510.5766@asgard.lang.hm>
Date:	Mon, 30 May 2011 15:41:31 -0700 (PDT)
From:	david@...g.hm
To:	"D. Jansen" <d.g.jansen@...glemail.com>
cc:	Theodore Tso <tytso@....edu>, Oliver Neukum <oneukum@...e.de>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Dave Chinner <david@...morbit.com>, njs@...ox.com,
	bart@...wel.tk
Subject: Re: [rfc] Ignore Fsync Calls in Laptop_Mode

On Mon, 30 May 2011, D. Jansen wrote:

> On Mon, May 30, 2011 at 10:53 PM,  <david@...g.hm> wrote:
>> On Mon, 30 May 2011, D. Jansen wrote:
>>
>>> On Mon, May 30, 2011 at 8:45 PM,  <david@...g.hm> wrote:
>>>> it doesn't take running a mail server, even a mail client will have the
>>>> same
>>>> risk. If you use POP for mail (a very common option) then you download
>>>> messages and tell the server to delete them. if you do not really save
>>>> them
>>>> (one fsync after they are all saved), then you can loose everything that
>>>> you
>>>> downloaded.
>>>
>>> Yes, I know. It's the same argument again and again. I understand not
>>> everybody wants this. But some do. Some prefer working 10-20% longer
>>> on battery (certainty) instead of possibly losing 5 % data
>>> (possibility) or losing all your data (possibility if you use laptop
>>> mode and the hard disk wakes up again and again and eventually wears
>>> out).
>>
>> those are some powerful numbers you are throwing around, can you back them
>> up?
>
> Yes. My netbook uses 4.8 W with hard disk off. 0.5 W more with hard
> disk on. A lot of the time, the display is off and the power
> consumption drops to about 3.5 W. I don't know how I got to 20 % atm.
> I can only do this if fsync is disabled. I tried before, but it would
> go on for every write. It died, very prematurely.

what are you doing that's causing so many fsyncs?

>>
>> do you really have so many fsync's going on that the disk spins up so much
>> that you would gain 10-20% battery life?
>
> Yes. Every autosave in LibreOffice triggers one. And I want autosave,
> but I want them in memory, not on disk.

are you sure that it's doing a fsync, not just a normal write? in laptop 
mode the drive will spin up occasionally for normal writes (to keep you 
from loosing too much data). I would expect that autosave is infrequent 
enough that (absent all other activity), each autosave will result in a 
spin-up

>> and what makes you think the extra spin-ups from fsyncs will cause your hard
>> drive to fail significantly earlier? (if you have a hard drive with a
>> limited number of spin-up cycles, you probably don't want to use laptop mode
>> at all)
>
> Experience, see above. Also, this is well described behavior. All hard
> disks are only designed to last a certain number of head loads and
> unloads. Spinning up and down even less.

right, but the trade-off of laptop mode is that it will do _more_ spin ups 
on the theory that it's better to spin up the drive once ina while than to 
keep it running. If you are looking at total power consumption this is the 
case, but if you are worried about load/unload cycles this is not the 
case.

the question I was asking is how do you know that fsync is the cause of 
these extra spin-ups? if you disable fsync, but still do the same number 
of writes, willyou really have far fewer spin-up events? I don't think 
that it would make nearly the difference ou think it is.

>> why do you think it's a possibility of loosing only 5% of data?
>
> Well, it really depends on the way your configure laptop mode. But in
> my case the laptop mode window is 20 minutes, the run time then is
> about 10-12 hrs. I can actually lose less than 5 % of the data created
> during the battery run time. It's a certain 1-1.5 hours (10-15 %) more
> work or a possible 20 mins (3.3 % actually) loss of work. I think it's
> a good deal.

as others point out, one missing write can cause you to loose a large 
chunk of data, not just the data being worked on during that time.

so if you end up with a corrupt file, you loose 100% of the file, not just 
the changes.

David Lang