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From: JMS@ARIZMIS.BITNET (You do their work, and they shall have good ...)
Newsgroups: comp.os.vms
Subject: RE: 6250 tape backup
Message-ID: <8706031526.AA24352@ucbvax.Berkeley.EDU>
Date: Tue, 2-Jun-87 20:09:00 EDT
Article-I.D.: ucbvax.8706031526.AA24352
Posted: Tue Jun  2 20:09:00 1987
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DNEIMAN%CARLETON.EDU@RELAY.CS.NET asks:

>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?).
>More info:  This drive may or may not be placed on the HSC; it may go on
>one of the other machines.  Is a 750 a reasonable place for a 6250 bpi drive?
>How about a 780?  What are the performance tradeoffs?

At the University of Arizona we have just converted from dual TU77s to
dual TU78s.  The driving CPU is a 780; the disks are RA81s on
an HSC50.  Our observation is that the 780 CPU is a bottleneck for using
BACKUP in the canonical fashion (with ECC).  We have not seen any real
significant speed increase in backups when moving from TU77s to TU78s.

Our backups were done as two-drive backups, and that made things a LOT
faster -- while one drive was rewinding, the other drive was spinning --
than normal, one-drive backups.  We never tried running dual backups
on two CPUs at the same time.  I suspect that the HSC50 would have
no trouble handling such a load.

The TU78s reduce operations time significantly, and make the tape library
a lot easier to deal with, but don't seem to run much faster due to the
CPU bottleneck.

I can't say for sure, but I would guess that you would find a TU78 on
a VAX 750 to run at about the same speed as your TU80s.  We don't use
the TU80s for BACKUP, except in emergency, but when we have, they
seem to run about 20 - 30% slower than the TU77s.

Here are some representative times:

Four RA81s -- BACKUP incremental on 3, full on 1:
BACKUP/BUF=65534/BLOCK=5 with CRC

TU77s:
2:30, 2:30, 2:15, 1:55, 2:00, 2:35, 1:55, 2:05, 2:25

TU78s:
1:45, 2:30, 2:15, 2:10, 3:00, 2:10

We will install an RH on an 8600 and put the drives on that next
week; I'll send out results if there is a significant improvement.

+-------------------------------+
| Joel M Snyder                 |            BITNET: jms@arizmis.BITNET
| Univ of Arizona Dep't of MIS  |          Internet: jms@mrsvax.mis.arizona.edu
| Tucson, Arizona 85721         |   Pseudo-PhoneNET: (602) 621-2748
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(I have gotten into trouble too many times to put any faith in disclaimers)
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