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From: BOLTHOUSE%MCOPN1@eg.ti.COM.UUCP
Newsgroups: comp.os.vms
Subject: 6250 tape backup
Message-ID: <8706041030.AA17840@ucbvax.Berkeley.EDU>
Date: Wed, 3-Jun-87 10:08:00 EDT
Article-I.D.: ucbvax.8706041030.AA17840
Posted: Wed Jun  3 10:08:00 1987
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>From: DNEIMAN%carleton.edu@RELAY.CS.NET
>
> (Information describing configuration:  2 750s, 1 780, 2 TU80s)
>
>Is is possible from the above information to determine whether and how much
>the 6250 tape drive will speed up backup (Is the bottle neck tape throughput,
>disk i/o, or cpu?).

All of this depends upon the command you're using for backups.  If you're
using /CRC (the default), your CPUs won't be able to keep up with a TU78,
for example.  (This according to DEC Storage folks at DECUS, Spring 1986).
If you're going to buy a TU81(+), you probably won't be able to get it to
stream continuously by using your 750s.  You *may* be able to get it to
stream using your 780, but I doubt it, especially if you have lots of
detached processes running around or users on the system during incremental
backups.

Also, BACKUP spends a bunch of time searching disks for files to back up.
If your disks are fragmented, then this time increases dramatically.  Using
the /FAST qualifier (and the requisite memory for the BACKUP process'
working set!) will ease this.

The upshot?   For incrementals, you're probably spending most of your time
doing disk I/O and waiting for the tape drive in stop/start mode.  I don't
think getting a TU81+/TA81 will help much there.  For /IMAGE backups, you
*might* get the tape drive to stream a little.  My recommendation is to
trade your 2 TU80s for a single TA78 if backup time is an issue.  If, on
the other hand, you're using cheap labor...maybe the costs of a TA78 are
too high.  In this case, get a TA81 and drive it off the HSC, and take the
performance hit.  (The HSC will at least tend to offload the VAX CPU).

You can always help yourself by keeping your disks contiguous.  And if you
don't care about your data, you can use things like /NOCRC, /BLOCK=65000
or RABBIT-5 to speed up your backups.  But I take the hit on backup speed
because I like knowing my customer's data are safe.

David L. Bolthouse
Texas Instruments Defense Electronics Information Systems VAX System Support
McKinney, TX

Ma Bell:  214.952.2059
Internet: bolthouse%mcopn1@ti-eg.com

Disclaimer:  My company's lawyers say you shouldn't take anything I say
             seriously.  Then again, when was the last time you took
             what a lawyer says seriously...