geom_sched usage

[Luigi Rizzo]

On Mon, Oct 18, 2010 at 09:43:28AM +0200, David Naylor wrote:
> Hi,
> 
> I've used geom_sched to some success.  Normally I do not notice anything but 
> today I was copying big files over a gigabit ethernet and my laptop was not 
> very responsive.  I loaded gsched and the responsiveness improved (although 
> still rather bad for anything requiring something from the HDD).  
> 
> Thank you for all this work :-)
> 
> Some questions I have:
> 
>  - with a gmirror should the gsched be attached to the underlying devices (aka 
> /dev/ad?) or to the mirror device (aka /dev/mirror/?)?

always attach as close as possible to the hardware.


>  - is there anyway to automatically attach gsched to a device on startup (i.e. 
> in rc.conf)?

no, you have to build some script yourself.

> 
>  - is there a way to prioritise random IO (vs sequential reads from big 
> files)?

no way to do that, but you can modify the quantum size and time to
let sequential reads get shorter chunks

kern.geom.sched.rr.quantum_kb: 8192
kern.geom.sched.rr.quantum_ms: 100
kern.geom.sched.rr.wait_ms: 10

e.g. on a laptop it might make sense to set
quantum_ms=50 and quantum_kb=2048

>  - gsched_as does not appear to be installed.  

true, gsched_as was just a proof of concept and gsched_rr
includes anticipation and round robin, so it is a superset of gsched_as

cheers
luigi