vfs.hirunningspace on a 3ware 8506

[Jim C. Nasby]

On Wed, Apr 21, 2004 at 12:55:07AM +0200, Uwe Doering wrote:
> Petri Helenius wrote:
> >Jim C. Nasby wrote:
> >
> >>Has anyone done any testing to see what value of vfs.hirunningspace is
> >>optimal for a 3ware 8506-8?
> >>
> >Do the 3ware controllers actually care about this value due to the 
> >onboard processing and cache? I thought all writes are satisfied 
> >immediately?
> 
> The controller itself doesn't care, but the kernel does.  With the 
> current implementation, the amount of memory associated with outstanding 
> read requests is subtracted from vfs.hirunningspace.  With many 
> concurrent read requests there is no reserve left for write operations, 
> so write performance can suffer substantially.
> 
> This balancing effect is actually intended in order to give read 
> requests some priority, but in high performance systems with fast, 
> caching raid controllers the default value of said variable is too low 
> and therefore poses a bottleneck.
 
Unfortunately, it seems the 8500 series only has 1.8MB of cache, so it
seems like the out-of-the-box setting of 1M may not be too far off.

Is it normally advisable to set vfs.hirunningspace = whatever the
controller's cache is?
-- 
Jim C. Nasby, Database Consultant                  jim at nasby.net
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