RFC: Changing default filesystem parameters for power management reasons

Eric Sandeen sandeen at redhat.com
Thu Nov 27 15:59:08 UTC 2008

Matthew Garrett wrote:
> I'd like F11 to be more useful for disk power management. This involves 
> tuning various parameters in order to reduce disk access. There are some 
> tradeoffs involved, so I'd like feedback before pushing much of this.
> The first is relatime. I've just pushed Ingo's smarter relatime code 
> towards upstream again. In this configuration atime will only be updated 
> if the current atime is either older than ctime or mtime, or if the 
> current atime is more than a day in the past. The amount of time 
> required before atime is updated will be a tunable, and a norelatime 
> mount parameter will be available to mount filesystems without this 
> behaviour. This shouldn't affect the behaviour of any applications.

I could be convinced of this, I think, although there were a few nagging
bugs w/ older Fedoras that seemed related to this change, and honestly
never got to the bottom of them.  But by and large Fedora already ran
this way w/ few problems in the past.

> The second is to increase the value of dirty_writeback_centisecs. This 
> will result in dirty data spending more time in memory before being 
> pushed out to disk. This is probably more controversial. The effect of 
> this is that a power interruption will potentially result in more data 
> being lost. It doesn't alter the behaviour of fsync(), so paranoid 

s/paranoid/proper/ :)

> applications will still get to ensure that their data is on disk. Of 
> course, it would also be helpful to stop applications generating dirty 
> pages where possible. This would obviously be reverted if the system 
> enters a critical power state.
> Thirdly, I'd like to enable laptop mode by default. The effect of this 
> is that any access that goes to disk will trigger an opportunistic 
> flushing of dirty data shortly afterwards. To an extent this mitigates 
> the change to dirty_writeback_centisecs, but there's obviously still 
> some increased chance of data loss.

I'll need to ponder these changes a bit more (and take another look at
laptop mode, it's been a while).

> The combination of these features should result in (on average) fewer 
> disk accesses and so (on average) should provide better performance. 

What are your plans to measure the results of these changes from power &
performance perspectives?  Also, tools to monitor what is causing disk
accesses might be good (see also Bug 454582 -  Tracker bug for
over-eager apps that won't let disks spin down).

Do you also have any plans for changing default disk spin-down times, or
would that be left to bios settings?  And if so, we should probably
monitor this for how it jives with the expected lifetime of a disk vs.
lifetime rating for spindown cycles.

The original laptop mode kit included specific knowledge about some
filesystem tuning parameters (commit times etc), is that part of your
plan?  Which filesystems will be recognized?


> There's a chance that some usage patterns will fall foul of this and 
> lose performance, so if we do this I'd like to do it sufficiently early 
> in the cycle that we can get real-world feedback.
> Any thoughts?

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