RFC: Changing default filesystem parameters for power management reasons

[Bill Nottingham]

Matthew Garrett (mjg at redhat.com) said: 
> 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.

Works for me.

> 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 
> 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'd be curious how this affects various workloads if we're changing
the global defaults. Were you planning on flipping the kernel defaults,
or just setting a default in sysctl.conf? (It occurs to me that laptop_mode
is horribly named.)

Bill