Messages in this thread | | | Date | Fri, 3 Apr 2009 02:06:00 +0100 | From | Matthew Garrett <> | Subject | Re: Ext4 and the "30 second window of death" |
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On Thu, Apr 02, 2009 at 05:55:11PM -0700, david@lang.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.
> >No. Ignoring fsync() makes it difficult for an application to > >inappropriately spin up a disk - but it also makes it *impossible* for > >an application to save data that it genuinely needs to. Doing this in > >kernel means that you have no granularity. You ignore the inappropriate > >fsync()s, but you also ignore the ones that are vitally important. I've > >no objection to the kernel supporting this functionality, but it should > >be /proc/sys/vm/fuck-my-data-harder rather than > >/proc/sys/vm/laptop-mode. > > > >Power management is a tradeoff. Sometimes providing correct > >functionality costs more than providing incorrect functionality. In > >general we strive to carry on providing applications the behaviour they > >expect even if it costs us more power - the alternative leads to users > >disabling power management functionality because they can't trust it. > >Throwing data away isn't an acceptable tradeoff for an extra three > >minutes of battery life for most users. > > I would agree with you if it was three minutes of battery life, but what > if it's an extra hour? (easily possible if the fsyncs make the difference > between the drive running all the time and waking up every 5 min for a few > seconds)
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.
-- Matthew Garrett | mjg59@srcf.ucam.org
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