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From: herzlich@NGP.UTEXAS.EDU (Larry Herzlich)
Newsgroups: comp.os.vms
Subject: Re: halve BACKUP times easily (Long reply)
Message-ID: <8707101441.AA05187@ngp.utexas.edu>
Date: Fri, 10-Jul-87 10:41:58 EDT
Article-I.D.: ngp.8707101441.AA05187
Posted: Fri Jul 10 10:41:58 1987
Date-Received: Sun, 12-Jul-87 14:15:00 EDT
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Since I've seen this recommendation multiple times, I think
it's time to respond.  I'm paranoid about backups.

>From: kai@uicsrd.CSRD.UIUC.EDU
>      Patrick Wolfe @ Kuck & Associates, Inc.
>Subject: halve BACKUP times easily
>Date: 7 Jul 87 01:16:00 GMT
>
>Using the following VMS/BACKUP switches can HALVE your cpu and elapsed time.
>	BACKUP/IMAGE/NOCRC/BUFFERS:5/BLOCKSIZE:16384  dra2:  msa0:dra2.bck
>..... So why have your VAX waste it's time doing double CRC checking?
>
>The /BUFFERS and /BLOCKSIZE switches help, and should probably be used even
>if you don't want to trust the /NOCRC switch.

At the Nashville DECUS, one of the VMS File System devolpers was very
adament about not turning off ECC in the BACKUP command line.  He did
point out that it depends on how paranoid you are about whether the
information got to the tape. There are a lot of places for the data to
get corrupted by the time it gets to the tape.  In a sense, the
hardware ECC is only checking whether the information in the tape
drive memory is sent out correctly.  The BACKUP Utility is your only
guarantee that the information got to the tape.

In addition, a VERIFY pass is important due to the way the information
gets recorded.  Particles on the tape get charged and remain at a
potential for a short time as the head passes over the media.  As the
backup continues, the tape charge potential drops down to the normal
level.  The VERIFY pass is checks to see that the information is
readable.

In an earlier Info-VAX message, Joel Schneider of the University of
Arizona recommended using this:
	>BACKUP/BUF=65534/BLOCK=5 with CRC

You can actually pull the tape off the reel with large buffers.  The
VMS developer recommends buffers of 32K or less and the default count
of 3.  The reason is the way VMS sends out pending buffers.  When the
physical EOT is reached, BACKUP continues to write the outstanding
QIO's.  With 65K buffers, you can end up writing a lot of information
past the EOT.


Larry Herzlich				herzlich@ngp.cc.utexas.edu
University of Texas at Austin		cccs001@utadnx  -- Bitnet
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