Path: utzoo!attcan!uunet!cs.utexas.edu!sdd.hp.com!hp-pcd!hpfcso!hpfcdj!kinsell
From: kinsell@hpfcdj.HP.COM (Dave Kinsell)
Newsgroups: comp.sys.hp
Subject: Re: HP-IB or SCSI disks for a 300 series?
Message-ID: <17330023@hpfcdj.HP.COM>
Date: 6 Nov 90 02:16:05 GMT
References: <1413@limbo.Intuitive.Com>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 38


>> If that's the case, then what's the fastest, most reliable, HP-IB 
>> disk available today, from HP or otherwise?  What's it list for?  

>All of HP's disks seem rather similiar in performance. 

I wouldn't agree with that, at least with the type of throughput tests
that you're using.  The older 795X[B|S] disks, both HP-IB and SCSI,
maxed out at 350 k/sec through the file system.  The newer C22XX disks
are enough faster that SCSI starts to make a big difference, giving 780
k/sec instead of 400.  The internal drive used on the 345 and 400t gives
950 k/sec on reads, but only 350 on writes.

The random performance doesn't show nearly that much spread, since they've
all got 16-18 ms random access times.  The C22XX drives spin at 4000 rpm,
which helps a bit on the latencies.

The real question is how you want to define "fast" disk drives.  There's
been a lot of attention focused on simple benchmarks that measure 
sustained throughput, but tracing of real workloads has shown that
this is not terribly important, given the amount of randomness in the
disk access patterns.  The Berkeley file system is capable of generating
highly sequential traffic, but most workloads don't show that.  For
example, demand load executables cause less total bytes to be read,
but the randomness in the disk traffic causes a rather large decrease
in the rate at which a Winchester can deliver the data.  If you're
swapping to the same disk used for file system, then the excursions back
and forth between the two regions can really degrade your file system
throughput.

The bottom line is that some improvement can be expected with SCSI, but
with most workloads, the gains aren't going to be breathtaking.  The
random access times tend to dominate the transaction, and people aren't
as disk bound as they sometimes believe.


-Dave Kinsell
 kinsell@hpfcmb.hp.com