Keep your hair on Bob

Ok so tyou know about drives, point taken.

I think you misinterpretted my mail.

I was merely trying to find out the maximum I/O the drive supports ,
phew.

Dave 




 

-----Original Message-----
From: bob944 [mailto:bob944 at attglobal.net] 
Sent: 13 October 2006 23:08
To: veritas-bu at mailman.eng.auburn.edu; Clooney, David
Subject: RE: [Veritas-bu] 9940B

> Was wondering whether anyone knows the max I/O buffer size for the STK

> 9940B ?
>  
> Trying to fine tune some media servers using the SIZE_DATA_BUFFERS and

> NUMBER_DATA_BUFFERS.
>  
> And was thinking the drive would obviously determine what the settings

> should be.

That's unlikely to be correct.  How does the drive subdivide its buffer
space (or does it?)?  How does it use it--wait for it to fill, then
compute and write, waiting for it to empty before accepting more data
(both unlikely)?  Would it wait to fill before looking to find out it
contained 64KB of zeros?

The point is that without knowing the drive firmware, we have no idea
how the drive uses its space--and if there were a performance advantage
to be had by matching buffer sizes, the vendor would certainly tell you
about it.  (BTW, the one modern drive firmware I have studied works
nothing like a fixed buffer, but much more like a round-robin queue,
optimized for continuous input of data and [de]compression at one "end"
while writing out the other.  Its size is relevant only for avoiding
over/under-run; it's rate that matters.)  Your objective is to never let
the drive run dry on write, and never let it fill on read.  Matching
buffer sizes doesn't do either.

Buffer tuning results for serverA in infrastructureA are often quite
different from best results for serverB/infrastructureB with the same
tape drives.  If there's a better way to determine them than
empirically, I'd like to know it.  The only correct answer IMO is to
open the Tuning Guide and do the work.  After you do it the right way,
you have the numbers to understand your infrastructure and predict the
results of changes to it.




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