kai@uicsrd.UUCP (07/07/87)
There was a item in this months DEC Professional Magazine that should be of
interest to VMS system managers and operators.
Using the following VMS/BACKUP switches can HALVE your cpu and elapsed time.
I tried this on a one of my VAX 750's Massbus disks (approx. 65 Mb) to a CDC
92185 tape drive at 6250 bpi, and cut the full disk backup elapsed time from
35 minutes to 17 minutes (on an unloaded system) and cputime used from 10
minutes to 5 minutes.
BACKUP/IMAGE/NOCRC/BUFFERS:5/BLOCKSIZE:16384 dra2: msa0:dra2.bck
The idea behind this is that BACKUP was written a long time ago, when 6250
bpi tape drives were non-existent or very expensive. 1600 bpi tape drives do
not have the hardware built in for performing CRC checking of the data,
however 6250 bpi tape drives DO. 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.
I compared this to our Raxco Rabbit-5 fast backup program (which I normally
use), and Rabbit-5 took 15 minutes to backup the same disk. Hardly worth the
$4000 price tag just to have a barely functional tape library manager (and
vaporware file locator).
Patrick Wolfe internet: pwolfe@kai.com
Kuck & Associates, Inc. uucp: {seismo,ihnp4,uiucuxc}!kailand!pwolfe
1808 Woodfield Dr. bitnet: pwolfe%kailand@uiucuxc
Savoy, IL 61874 csnet: pwolfe%kailand%uxc@uiuc.csnetherzlich@NGP.UTEXAS.EDU (Larry Herzlich) (07/10/87)
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 Computation Center User Services Division (512) 471-3241