SUMMARY : Allocating swap space on HSZ40 controled unit

From: Jean-Francois Blanchet <jfblanchet_at_videotron.ca>
Date: Mon, 03 Mar 1997 13:56:51

Thanks again to:

Cliff Krieger <ckrieger_at_latrade.com>
alan_at_nabeth.cxo.dec.com

Here is the original question....
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Hi,
I have to allocate more swap space on my system, currently the swap space
are allocated on the internal disk of my Alpha 2100.
1- Is it a good thing to allocate swap space on HSZ40 controled unit?
   (on the same bus I have unit with intensive database i/o)
2- If i can, I enable or disable WRITE-BACK cache and READ_CACHE on that
storage unit?
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Cliff Krieger <ckrieger_at_latrade.com>
:1- Is it a good thing to allocate swap space on HSZ40 controled unit?
:   (on the same bus I have unit with intensive database i/o)
As a general rule, people purchase HSZ40s because they have an
application that needs high reliability and/or speed.  In the interest
of speed, I would have the swap partition on a disk by itself, and
ideally on a bus by itself.  I don't think I would ever put it on an
HSZ, but there is no technical inhibition against it.
:2- If i can, I enable or disable WRITE-BACK cache and READ_CACHE on that
:storage    unit?
If you do it, I would disable all the caching so that you are not taking
away from the performance of the other applications that use the HSZ.
At the very least, the write-back cache would be wasted because swap
data is not crititcal and does not need to survive a crash.  One can
argure either way about the read cache.
-cliff
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alan_at_nabeth.cxo.dec.com
	Are you expecting to see lots page/swap I/O, or do you just
	need the space for the virtual memory needs of the system/
	application?  If the existing bus to the HSZ40 is saturated
	then adding significant page/swap I/O is a bad idea.  And
	if you're going to spend money to improve page/swap performance
	you're better off getting more memory if you can.  If you
	don't have much paging I/O now and there is bandwidth left
	on the bus, spreading the I/O is a good idea.
	re: Write cache
	Always a good idea.  For small paging I/Os, it will let
	them complete quickly.  For large swapping I/Os, it won't
	be worse than writing to uncached disk.
	re: Read cache
	A good question.  See what utilities, if any, the HSZ has
	for monitoring cache hits.  Try it with the read-cache
	enabled and see if it is having any affect.  If not, disable
	it because it adds latency to the I/O path.
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Jeff

Received on Mon Mar 03 1997 - 20:21:24 NZDT