Write efficiency

I'd be grateful for some help in understanding the write efficiency parameter 
in the Disk Activity section of the Performance Monitor Component Report.  We 
have an S30, 1Gb main storage, 18 4Gb disks on 2 IOP's, running a client/server 
ERP application.  Minimal green screen.
We are getting Write Efficiency figures per disk of 1 to 45%.   The single 
average figure shown in the summary section below is typically 40%, (overall 
disk utilization over the same period is 8 to 10%).  

My question is what do these write efficiency figures mean in practical terms, 
i.e.
How relevant/important are they?
Is 40% bad or good?
If bad, what effect are they having on overall system performance?
How can they be improved - system parameter tweaks, more Main Storage, more 
IOP's, more disks?
Is there a "wall" at which performance falls off drastically?  

We have had a scout through the manuals but would appreciate any pointers to 
relevant sections.  

Thanks, John Lewis-Crosby

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