RFC: Changing default filesystem parameters for power management reasons

[Matthew Garrett]

On Thu, Nov 27, 2008 at 09:59:08AM -0600, Eric Sandeen wrote:

> 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).

Power-wise, I have measuring equipment here. Performance is obviously 
harder - I suspect synthetic benchmarks will get much the same 
performance as usual, so that might be down to waiting to see if users 
complain.

It would be nice to have the kernel export disk access via a socket or 
something rather than the currently available debug option, which is to 
dump to dmesg which then triggers log writes which triggers more 
messages and fail occurs. I had a handwavy patch to do that, but we 
probably want a good way of exposing that information to userspace.

> 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.

Yes, the long-term plan involves allowing drive spindown. I'm hoping to 
do this adaptively to let us avoid the spinup/down tendancies a static 
timeout provides, but you're right that monitoring SMART information 
would be a pretty important part of that. I lean towards offering it on 
desktops and servers, but not enabled by default.

> 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?

Mm. My recollection is that ext3 and xfs had easy to access tuning to 
help in this respect. Changing the kernel defaults would be one option 
there, or alternatively we could update fstab?

-- 
Matthew Garrett | mjg59 at srcf.ucam.org