[laptop-mode] Enabling Laptop Mode Activate Pdflush Every 3-4 Seconds

[Bart Samwel]

Casey McGinty wrote:
> 
> 
> On Thu, May 28, 2009 at 9:43 PM, Bart Samwel <bart at samwel.tk 
> <mailto:bart at samwel.tk>> wrote:
> 
>     Hi Casey,
> 
> 
>     The thing is, laptop mode does a sync *across all file systems*. And
>     that causes disk activity even on file systems that aren't being used
>     much. Combine this with a laptop mode sync loop (as you are
>     experiencing) and it becomes very annoying. :-/
> 
> 
> Well, that sounds like a fundamental flaw to me. I'm not sure why you 
> would want to cause a write on a file system that has no data to write.

The assumption is that "sync" on a non-dirty fs is a no-op, so we can 
simply sync all file systems. Apparently there are file systems that 
break this, and then you get this kind of behaviour. :-/ I might want to 
take this up with the fs developers...

>     I personally haven't verified this, so I will have to go and build such
>     a set-up myself to reproduce the problem. I strongly suspect that LVM
>     writes data after the sync is complete, which causes laptop mode to
>     trigger a re-sync after two seconds.
> 
> 
> This seems logical. A more likely scenario might be that the write is 
> going to update the timestamp or some flag on the RAID superblock to 
> indicate that the devices in the array are consistent. If this is true, 
> then laptop-mode is clearly broken for systems with RAID.

That sounds like a very plausible scenario indeed. The RAID superblock 
writes are independent of the fs sync, so they will most likely occur 
after the fs sync has completed.

> For now it looks like I've reached the limitations of "laptop-mode" on 
> my system. I'll check back in periodically to see how the progress is 
> going. Thanks for all the help and advice.

No problem! Hopefully we can resolve this at some point, and we'll let 
you know...

Cheers,
Bart