Networker

Re: [Networker] How to tell how much physical tape is being used?

2007-04-24 15:44:34
Subject: Re: [Networker] How to tell how much physical tape is being used?
From: Doug Brown <Doug.Brown AT ABBOTT DOT COM>
To: NETWORKER AT LISTSERV.TEMPLE DOT EDU
Date: Tue, 24 Apr 2007 14:41:01 -0500

George

Keep in mind too that if you use the incremental functionality of RMAN, then only blocks that have changed since the previous level - 1 will be backed up.   And RMAN only backs up blocks used in the database period in a level 0 ( full ) backup.   So if your tablespace is sized at 100GB and the objects inside are only using 10GB then you're only using 10GB of tape.



George Sinclair <George.Sinclair AT NOAA DOT GOV>
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04/24/2007 02:35 PM
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[Networker] How to tell how much physical tape is being used?





Is there a way to determine how much physical tape space is being used
by a backup?

For example, lets suppose I run an Oracle backup using RMAN with NMO,
and the RMAN script uses Oracle's compression feature (compressed), and
let's say the backup is a level full and creates 5 save sets at around
200-300 MB each (each save set has say 4-6 files each). Next, I run the
same backup, but this time I don't use the 'compressed' option in the
RMAN script, and maybe I get 5 save sets again, but this time they're
much larger, say 1 GB each. I recover everything in both examples, and
the recovered database is exactly the same as far as the number of files
and their sizes. Well, hardware compression is enabled on the drives, so
the data is getting compressed both times, and in the second example,
while I may be backing up approx. 5 GB of data, I'm probably not using 5
GB worth of tape space due to hardware compression, so it is possible
that both examples result in the same amount of physical tape space
being used? Maybe?

Is there a way to tell which method is actually using more tape or if
they're close?

I can query for the 'mediafile' numbers for the save sets, and I notice
that they always start 2 beyond the previous one, but a given saveset
could be of any size so this doesn't really tell me how much tape is
being used, right?

Here's why I ask such a goofy (stupid?) question. OK, we ran this Oracle
backup (level full) with the 'compressed' option, and we noticed that
the backup size was very small (kinda nice!), but as a result, the drive
speeds were very slow. This is not surprising given that we probably
were underfeeding the drives since the client was sending less data due
to the Oracle compression. Next, we run the same thing but without the
'compressed' option, and now the drives scream (again, not surprising
given that the client now has more data to send), but the backup sizes
were way larger. Overall, however, the backup completion times were
reasonably similar. I'm wondering if maybe in the second example that
because the data was not already compressed that the drives shrunk it
down and that maybe it's not really taking up any more space on the tape
than the same data in the first run. We recovered both sets, and the
total number of recovered database files and their sizes are the same in
both cases.  How can I determine, therefore, which method used less tape?

Thanks.

George

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George Sinclair - NOAA/NESDIS/National Oceanographic Data Center
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