From:	CRDGW2::CRDGW2::MRGATE::"SMTP::CRVAX.SRI.COM::RELAY-INFO-VAX" 11-AUG-1989 07:42
To:	MRGATE::"ARISIA::EVERHART"
Subj:	Re: 8 mm tape drive performance

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Date: 11 Aug 89 10:59 -0500
From: John Edgecombe <edgecombe%ccrs.cdn@relay.ubc.ca>
To: <info-vax@kl.sri.com>
Message-Id: <74*edgecombe@ccrs.cdn>
Subject: Re: 8 mm tape drive performance

>Greetings. I am trying to achieve maximum performance on writes to an
>Exabyte 8 mm drive on an MTI controller, attached to a uVAX 3600 (2.7 vup?).
>...

This report was prepared by Bill Park of Canada Centre for Remote Sensing:
Correspondence should be directed to me, as he is not on the net.

------------------------------------------------------------------------

                          8mm tape drive performance
                            Status as of Feb. 1989

             The EXABYTE-8200 8mm cartridge (246KB/S  2.3GB  Capacity)
        is  connected  to  a  DIALOG DU142 controller to a UNIBUS on a
        VAX-750 running VMS 4.7.  The EXABYTE is used for  backup  and
        for  storing  data  stripped  in real time from a high density
        instrument tape where for a given run the record size and  i/o
        rates  are  fixed  and  maximized  subject  to the CPU and i/o
        bandwidths.

             We  have  measured  the  performance   capabilities   and
        investigated the peculiarities of the EXABYTE:-

        1.  Because the EXABYTE drive and not  the  controller  has  a
            256k  internal  buffer  memory it is possible to exceed in
            burst mode the maximum  time  averaged  transfer  rate  of
            246KB/sec  and  there  is  little  interference if several
            drives are on the same controller.  We have achieved burst
            mode  i/o  rates of 277KB/sec using QIOWs (inefficient) of
            7020byte records.

        2.  When the tape in mounted or rewound there will be a  30sec
            delay  to  bring the tape to the R/W heads before data can
            be written to the tape.

        3.  Writing a file mark takes 10secs.

        4.  There is a startup problem writing data in real time which
            limits  the  writes/sec  and  transfer  rate but once this
            settling in period one can achieve close to the  240KB/sec
            transfer rate.

                 The table below indicates the effective i/o  transfer
            rate that can be guaranteed for real time applications due
            to the EXABYTE startup problems and VMS overhead which  is
            thought  to  be  the  main  problem when the writes/sec is
            greater than about 25.  There are some  outriders  to  the
            curve  in  the  table  below,  eg  we  can  transfer about
            220KB/sec at 50 write/sec but anything slightly  different
            will fail.  We have no explanation for this phenomenon.

                KB/sec  Write operations/sec
                250     10
                240     15
                180     20
                100     25
                 70     30
                 60     35
                 40     40

                 It should be remembered that when we  read  from  the
            real  time  device  at  the  same  rate as we write to the
            EXABYTE.   80  i/o  operations  per  second  is  quite  an
            achievment  for  a  VAX-750!  The real time device and the

                                                                Page 2


            EXABYTE HAVE to be on seperate UNIBUS for it all to work.

        5.  Reading 7020byte records from the EXABYTE using  QIOWs  we
            have  measured  30 read operations/sec (210KB/sec averaged
            over many minutes) providing you  specify  the  number  of
            bytes  to  read  as  7020.   If  you specify 7022 then the
            reads/sec drops to 14 (98KB/sec)!  This phenomenon is also
            exhibited  with  a MASSBUS TU78 125IPS CCT drive where the
            reads/sec drops from 56 to 26 but does not occur when  the
            EXABYTE  is  driven by a TD controller VIKING UTO where we
            achieve 22r/sec (154KB/s) max.  I have been  warned  about
            possible  protocol problems with the latter controller but
            have no details on what these are.

        6.  Appending or backskipping files or records then attempting
            to overwrite will not work.

        7.  At 7020byte records we can store 1.95GB on one cartridge.

        8.  The measured rewind time is 2 minutes on a full tape.

        9.  Mount /foreign takes about 1 minute.

       10.  SET MAGTAPE CCT:/SKIP=END takes 2.5hours on a full tape of
            7020byte records.

       11.  We have experienced no problems using the  EXABYTE  drives
            for backup purposes.

                            Status as of AUG. 1989

        1.  Some changes have been made to the controller and we  have
            upgraded to VMS 5.1.  The overall performance of the drive
            seems to have degraded a few percent.

        2.  Sometimes the read  i/o  rate  degrades  by  an  order  of
            magnitude for reasons that are not understood.  This seems
            to be cured by turning the computer power off and on.

        3.  BACKUP/list of a single 157000  block  file  yields  about
            100KB/sec

-- john

John Edgecombe			<edgecombe@ccrs.cdn>
Canada Centre for Remote Sensing
Centre Canadien de Teledetection
Ottawa, Ontario

