vfs.hirunningspace on a 3ware 8506

Uwe Doering gemini at geminix.org
Tue Apr 20 15:55:16 PDT 2004

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.

Uwe Doering         |  EscapeBox - Managed On-Demand UNIX Servers
gemini at geminix.org  |  http://www.escapebox.net

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