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From: kinsell@hpfcdj.HP.COM (Dave Kinsell)
Newsgroups: comp.sys.hp
Subject: Re: Disk performance HP-UX 6.5
Message-ID: <17330009@hpfcdj.HP.COM>
Date: 16 Dec 89 01:15:29 GMT
References: <2437@ifi.uio.no>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 49


{Sorry for the repost, but this likely didn't make it out to the public net}


>While testing the CDC Wren IV disk some dramatic differences between HP and SUN
>turned up.  Running the dd command illustrates the point. We reads 2000 blocks
>of 8K size from the disk with the command

>     dd if=/dev/<block-special> of=/dev/null bs=8k count=2000

>and the time (measured with /bin/time) it takes do this is   

>Sun 3/80 with SunOS 4.03    real:  18.4   sys: 8.2
>HP 9000/370 with HP-UX 6.5  real: 144.8   sys: 9.1.

Using dd with the block special device file isn't doing what you think
it's doing.  For reasons I don't understand, the physical I/O is broken
into 2 Kbyte chunks, at least on the HP system.  Note that the man page
for dd says that the blocksize declaration works only for raw I/O.

The big factor in the performance difference is that the Wren IV has
readahead which must be specifically enabled.  Since it's not a
supported drive on the HP system, it doesn't get turned on.  It must be
getting turned on with the SUN system.

You're sending short, consecutive read requests to the disk, which is
exactly where readahead shows the biggest improvement.  However, it is
not at all representative of file system or swap activity.

Without readahead, it will take slightly more than one revolution to
get each 2K of data (one latency, plus skipping the previously read 2K):

   2K/18ms = 114 k/sec

Your results: 

   8K*2000/144.8 = 113 k/sec

The Wren IV is a ZBR technology drive, which means the data rate changes
significantly depending on what cylinder is being read.  This complicates
using it for file system benchmarking.


-Dave Kinsell
 use kinsell@hpfcmb.hp.com


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