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Old 11-27-2008, 02:33 PM
Matthew Garrett
Default RFC: Changing default filesystem parameters for power management reasons

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.

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.

The combination of these features should result in (on average) fewer
disk accesses and so (on average) should provide better performance.
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?
Matthew Garrett | mjg59@srcf.ucam.org

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